485BPOS

1933 Act File No. 002-62797
1940 Act File No. 811-02871

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
FORM N-1A
REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933	x
Pre-Effective Amendment No.	o
Post-Effective Amendment No. 50	x
and/or
REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940	x
Post-Effective Amendment No. 49	x

LORD ABBETT DEVELOPING GROWTH FUND, INC.

(Exact Name of Registrant as Specified in Charter)

	90 Hudson Street, Jersey City, New Jersey	07302-3973
	(Address of Principal Executive Office)	(Zip Code)
Registrant’s Telephone Number, including Area Code: (800) 201-6984

Thomas R. Phillips, Esq.

Vice President and Assistant Secretary

90 Hudson Street, Jersey City, New Jersey 07302-3973

(Name and Address of Agent for Service)

It is proposed that this filing will become effective (check appropriate box)
x	immediately upon filing pursuant to paragraph (b)
o	on (date) pursuant to paragraph (b)
o	60 days after filing pursuant to paragraph (a)(1)
o	on (date) pursuant to paragraph (a)(1)
o	75 days after filing pursuant to paragraph (a)(2)
o	on (date) pursuant to paragraph (a)(2) of Rule 485.
	If appropriate, check the following box:
o	this post-effective amendment designates a new effective date for a previously filed post-effective amendment.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, and the Investment Company Act of 1940, as amended, the Registrant certifies that it meets all of the requirements for effectiveness to its Registration Statement under Rule 485(b) under the Securities Act and had duly caused this Post-Effective Amendment to its Registration Statement to be signed on its behalf by the undersigned, duly authorized, in the City of Jersey City, and State of New Jersey as of the 18^th day of December, 2012.

LORD ABBETT DEVELOPING GROWTH FUND, INC.
	BY:	/s/ Thomas R. Phillips
		Thomas R. Phillips
		Vice President and Assistant Secretary
	BY:	/s/ Joan A. Binstock
		Joan A. Binstock
		Chief Financial Officer and Vice President

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

Signatures		Title		Date
/s/ Robert S. Dow*	Chairman, Chief Executive		December 18, 2012
Robert S. Dow	Officer and Director
/s/ Daria L. Foster*	President and Director		December 18, 2012
Daria L. Foster
/s/ E. Thayer Bigelow*	Director		December 18, 2012
E. Thayer Bigelow
/s/ Evelyn E. Guernsey*	Director		December 18, 2012
Evelyn E. Guernsey
/s/ Robert B. Calhoun, Jr.*	Director		December 18, 2012
Robert B. Calhoun, Jr.
/s/ Julie A. Hill*	Director		December 18, 2012
Julie A. Hill
/s/ Franklin W. Hobbs*	Director		December 18, 2012
Franklin W. Hobbs
	Director		__________________
James M. McTaggart
/s/ Thomas J. Neff*	Director		December 18, 2012
Thomas J. Neff
/s/ James L .L. Tullis*	Director		December 18, 2012
James L. L. Tullis

* BY:	/s/ Thomas R. Phillips
	Thomas R. Phillips
	Attorney-in-Fact

POWER OF ATTORNEY

          Each person whose signature appears below on this Registration Statement hereby constitutes and appoints Lawrence H. Kaplan, Lawrence B. Stoller, John K. Forst, and Thomas R. Phillips, each of them, with full power to act without the other, his or her true and lawful attorney-in-fact and agent, with full power of substitution and resubstitution, for him or her and in his or her name, place and stead, in any and all capacities (until revoked in writing) to sign any and all Registration Statements of each Fund enumerated on Exhibit A hereto for which such person serves as a Director/Trustee (including Registration Statements on Forms N-1A and N-14 and any amendments thereto), and to file the same, with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents, and each of them, full power and authority to do and perform each and every act and thing ratifying and confirming all that said attorneys-in-fact and agents or any of them, or their or his or her substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

          Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

Signatures		Title		Date
/s/ Robert S. Dow	Chairman, CEO		April 19, 2012
Robert S. Dow	and Director/Trustee
/s/ Daria L. Foster	President and		April 19, 2012
Daria L. Foster	Director/Trustee
/s/ E. Thayer Bigelow	Director/Trustee		April 19, 2012
E. Thayer Bigelow
/s/ Robert B. Calhoun, Jr.	Director/Trustee		April 19, 2012
Robert B. Calhoun, Jr.
/s/ Evelyn E. Guernsey	Director/Trustee		April 19, 2012
Evelyn E. Guernsey
/s/ Julie A. Hill	Director/Trustee		April 19, 2012
Julie A. Hill
/s/ Franklin W. Hobbs	Director/Trustee		April 19, 2012
Franklin W. Hobbs
/s/ Thomas J. Neff	Director/Trustee		April 19, 2012
Thomas J. Neff
/s/ James L .L. Tullis	Director/Trustee		April 19, 2012
James L. L. Tullis

EXHIBIT A

Lord Abbett Affiliated Fund, Inc.

Lord Abbett Bond-Debenture Fund, Inc.

Lord Abbett Developing Growth Fund, Inc.

Lord Abbett Equity Trust

Lord Abbett Global Fund, Inc.

Lord Abbett Investment Trust

Lord Abbett Mid Cap Stock Fund, Inc.

Lord Abbett Municipal Income Fund, Inc.

Lord Abbett Research Fund, Inc.

Lord Abbett Securities Trust

Lord Abbett Series Fund, Inc.

Lord Abbett Stock Appreciation Fund

Lord Abbett U.S. Government & Government Sponsored Enterprises Money Market Fund, Inc.

EXHIBIT INDEX

Exhibit No.	Description
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/

Objective [Heading]

rr_ObjectiveHeading

INVESTMENT OBJECTIVE

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The Fund’s investment objective is long-term growth of capital through a diversified and actively managed portfolio consisting of developing growth companies, many of which are traded over the counter.

Expense [Heading]

rr_ExpenseHeading

FEES AND EXPENSES

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and certain members of your family invest, or agree to invest in the future, at least $50,000 in the Lord Abbett Family of Funds. More information about these and other discounts is available from your financial professional and in “Sales Charge Reductions and Waivers” on page 24 of the prospectus and “Purchases, Redemptions, Pricing, and Payments to Dealers” on page 8-1 of the statement of additional information (“SAI”).

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

Shareholder Fees (Fees paid directly from your investment)

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses (Expenses that you pay each year as a percentage of the value of your investment)

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover.

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in the annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 195.11% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

195.11%

Expense Breakpoint Discounts [Text]

rr_ExpenseBreakpointDiscounts

You may qualify for sales charge discounts if you and certain members of your family invest, or agree to invest in the future, at least $50,000 in the Lord Abbett Family of Funds.

Expense Breakpoint, Minimum Investment Required [Amount]

rr_ExpenseBreakpointMinimumInvestmentRequiredAmount

$ 50,000

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

This amount has been updated from fiscal year amounts to reflect current fees and expenses.

Expense Example [Heading]

rr_ExpenseExampleHeading

Example

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund at the maximum sales charge, if any, for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year, that dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. The example assumes a deduction of the applicable contingent deferred sales charge (“CDSC”) for the one-year, three-year, and five- year periods for Class B shares and for the one-year period for Class C shares. Class B shares automatically convert to Class A shares after approximately eight years. The expense example for Class B shares for the ten-year period reflects the conversion to Class A shares. The first example assumes that you redeem all of your shares at the end of the periods. Although your actual costs may be higher or lower, based on these assumptions, your costs (including any applicable CDSC) would be as shown below. The second example assumes that you do not redeem and instead keep your shares.

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

If Shares Are Redeemed

Expense Example, No Redemption, By Year, Caption [Text]

rr_ExpenseExampleNoRedemptionByYearCaption

If Shares Are Not Redeemed

Strategy [Heading]

rr_StrategyHeading

PRINCIPAL INVESTMENT STRATEGIES

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

To pursue its objective, the Fund invests primarily in the common stocks of companies demonstrating above-average, long-term growth potential. The Fund seeks to identify companies that it believes are strongly positioned in the developing growth phase, which the Fund defines as the period of swift development after a company’s start-up phase when growth occurs at a rate rarely equaled by established companies in their mature years.

Under normal market conditions, the Fund invests at least 65% of its net assets in equity securities of small companies. In selecting investments, the Fund may invest in U.S. and foreign (which may include emerging market) companies. Foreign companies may be traded on U.S. or non-U.S. securities exchanges, may be denominated in the U.S. dollar or other currencies, and may include American Depositary Receipts (“ADRs”). Although the Fund is diversified across many industries and sectors, its assets may, from time to time, be overweighted or underweighted to certain industries and sectors relative to its benchmark index. The Fund’s principal investments include the following types of securities and other financial instruments:

• Equity securities usually include common stocks, but also may include preferred stocks, convertible securities, and equity interests in trusts, partnerships, and limited liability companies. The Fund also may invest in securities that are tied to the price of stock, including rights and convertible debt securities.

• Small companies having a market capitalization at the time of purchase that falls within the market capitalization range of companies in the Russell 2000® Index.

• Growth companies that the Fund believes exhibit faster-than-average gains in earnings and have the potential to continue profit growth at a high level.

The Fund may engage in active and frequent trading of its portfolio securities.

The Fund generally will sell a security when the Fund believes the security is less likely to benefit from the current market and economic environment, shows signs of deteriorating fundamentals, or has reached its valuation target, among other reasons. The Fund seeks to remain fully invested in accordance with its investment objective; however, in response to adverse economic, market or other unfavorable conditions, the Fund may invest its assets in a temporary defensive manner.

Risk [Heading]

rr_RiskHeading

PRINCIPAL RISKS

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

As with any investment in a mutual fund, investing in the Fund involves risk, including the risk that you may receive little or no return on your investment. When you redeem your shares, they may be worth more or less than what you paid for them, which means that you may lose a portion or all of the money you invested in the Fund. The principal risks of investing in the Fund, which could adversely affect its performance, include:

• Portfolio Management Risk: If the strategies used and securities selected by the Fund’s portfolio management fail to produce the intended result, the Fund may suffer losses or underperform other funds with the same investment objective or strategies, even in a rising market.

• Equity Risk: Common stocks and other equity securities, as well as equity-like securities such as convertible bonds, may experience significant volatility. Such securities may fall sharply in response to adverse events affecting overall markets, a particular industry or sector, or an individual company’s financial condition.

• Small Company Risk: The Fund invests primarily in equity securities of small companies, which typically involve greater investment risks than larger companies. Small companies may have limited management experience or depth, limited ability to generate or borrow capital needed for growth, and limited products or services or operate in markets that have not yet been established. Accordingly, small company securities tend to be more sensitive to changing economic conditions and tend to be more volatile and less liquid than equity securities of larger companies.

• Growth Investing Risk: The Fund uses a growth investing style, which may be out of favor or may not produce the best results over short or longer time periods. In addition, growth stocks tend to be more volatile than slower-growing value stocks.

• Foreign Company Risk: The Fund’s investments in foreign (including emerging market) companies and in U.S. companies with economic ties to foreign markets generally involve special risks that can increase the likelihood that the Fund will lose money. For example, as compared with companies organized and operated in the U.S., these companies may be more vulnerable to economic, political, and social instability and subject to less government supervision, inadequate regulatory and accounting standards, and foreign taxes. In addition, the securities of foreign companies also may be subject to inadequate exchange control regulations, higher transaction and other costs, reduced liquidity, and delays in settlement to the extent they are traded on non-U.S. exchanges or markets. Emerging market securities generally are more volatile than other foreign securities, and are subject to greater liquidity, regulatory, and political risks.

• Industry/Sector Risk: To the extent the Fund overweights a single market sector or industry relative to its benchmark index, it can accumulate relatively large positions in a single issuer, industry, or sector. As a result, the Fund’s performance may be tied more directly to the success or failure of a relatively smaller or less diversified group of portfolio holdings.

• High Portfolio Turnover Risk: High portfolio turnover (more than 100%) may result in increased brokerage fees or other transaction costs, reduced investment performance, and higher taxes.

An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. For more information on the principal risks of the Fund, please see the “More Information About the Fund – Principal Risks” section in the prospectus.

Risk Lose Money [Text]

rr_RiskLoseMoney

As with any investment in a mutual fund, investing in the Fund involves risk, including the risk that you may receive little or no return on your investment. When you redeem your shares, they may be worth more or less than what you paid for them, which means that you may lose a portion or all of the money you invested in the Fund.

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

PERFORMANCE

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The bar chart and table below provide some indication of the risks of investing in the Fund by illustrating the variability of the Fund’s returns. Each assumes reinvestment of dividends and distributions. The Fund’s past performance, before and after taxes, is not necessarily an indication of how the Fund will perform in the future.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

The bar chart and table below provide some indication of the risks of investing in the Fund by illustrating the variability of the Fund's returns.

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

888-522-2388

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.lordabbett.com

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

The Fund's past performance, before and after taxes, is not necessarily an indication of how the Fund will perform in the future.

Bar Chart [Heading]

rr_BarChartHeading

Bar Chart (per calendar year) - Class A Shares*

Bar Chart Narrative [Text Block]

rr_BarChartNarrativeTextBlock

The bar chart shows changes in the performance of the Fund's ClassA shares from calendar year to calendar year. This chart does not reflect the sales charge applicable to ClassA shares. If the sales charge were reflected, returns would be lower. Performance for the Fund's other share classes will vary due to the different expenses each class bears. Updated performance information is available at www.lordabbett.com or by calling 888-522-2388.

Bar Chart Does Not Reflect Sales Loads [Text]

rr_BarChartDoesNotReflectSalesLoads

This chart does not reflect the sales charge applicable to Class A shares. If the sales charge were reflected, returns would be lower.

Bar Chart Footnotes [Text Block]

rr_BarChartFootnotesTextBlock

* The total return for the Fund’s Class A shares for the nine-month period from January 1, 2012 to September 30, 2012 was +15.22%.

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

Best Quarter 2nd Q ‘09 +24.37% Worst Quarter 4th Q ’08 -28.60%

Year to Date Return, Label

rr_YearToDateReturnLabel

The total return for the Fund's Class A shares for the nine-month period from January 1, 2012 to September 30, 2012 was +15.22%.

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2012

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

15.22%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Best Quarter

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

24.37%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Worst Quarter

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2008

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(28.60%)

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Total Returns (for the periods ended December 31, 2011)

Performance Table Does Reflect Sales Loads

rr_PerformanceTableDoesReflectSalesLoads

The Fund's average annual total returns include applicable sales charges.

Index No Deduction for Fees, Expenses, Taxes [Text]

rr_IndexNoDeductionForFeesExpensesTaxes

(reflects no deduction for fees, expenses, or taxes)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

The after-tax returns of Class A shares included in the table below are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

The after-tax returns shown are not relevant to investors who hold their Fund shares through tax- deferred arrangements such as 401(k) plans or Individual Retirement Accounts ("IRAs").

Performance Table One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

After-tax returns for other share classes are not shown in the table and will vary from those shown for Class A shares.

Performance Table Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

In some cases, the return after taxes on distributions and sale of Fund shares may exceed the return before taxes due to a tax benefit resulting from realized losses on a sale of Fund shares at the end of the period that is used to offset other gains.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

The table below shows how the Fund’s average annual total returns compare to the returns of a securities index. The Fund’s average annual total returns include applicable sales charges.

The after-tax returns of Class A shares included in the table below are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. In some cases, the return after taxes on distributions and sale of Fund shares may exceed the return before taxes due to a tax benefit resulting from realized losses on a sale of Fund shares at the end of the period that is used to offset other gains. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their Fund shares through tax- deferred arrangements such as 401(k) plans or Individual Retirement Accounts (“IRAs”). After-tax returns for other share classes are not shown in the table and will vary from those shown for Class A shares.

Russell 2000® Growth Index (reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(2.91%)

5 Years

rr_AverageAnnualReturnYear05

2.09%

10 Years

rr_AverageAnnualReturnYear10

4.48%

Life of Class

rr_AverageAnnualReturnSinceInception

0.33%

Inception Date for Performance

rr_AverageAnnualReturnInceptionDate

Sep. 28, 2007

Class A

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

5.75%

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

none

^[1]

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.35%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.12%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

683

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

911

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

1,156

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

1,860

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

683

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

911

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

1,156

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

1,860

Annual Return 2002

rr_AnnualReturn2002

(29.54%)

Annual Return 2003

rr_AnnualReturn2003

40.06%

Annual Return 2004

rr_AnnualReturn2004

6.00%

Annual Return 2005

rr_AnnualReturn2005

11.87%

Annual Return 2006

rr_AnnualReturn2006

12.45%

Annual Return 2007

rr_AnnualReturn2007

35.83%

Annual Return 2008

rr_AnnualReturn2008

(47.50%)

Annual Return 2009

rr_AnnualReturn2009

47.03%

Annual Return 2010

rr_AnnualReturn2010

36.54%

Annual Return 2011

rr_AnnualReturn2011

(1.66%)

1 Year

rr_AverageAnnualReturnYear01

(7.32%)

5 Years

rr_AverageAnnualReturnYear05

5.83%

10 Years

rr_AverageAnnualReturnYear10

5.73%

Class A | After Taxes on Distributions

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(8.18%)

5 Years

rr_AverageAnnualReturnYear05

4.97%

10 Years

rr_AverageAnnualReturnYear10

4.86%

Class A | After Taxes on Distributions and Sale of Fund Shares

Risk/Return:

rr_RiskReturnAbstract

1 Year

rr_AverageAnnualReturnYear01

(3.65%)

5 Years

rr_AverageAnnualReturnYear05

4.70%

10 Years

rr_AverageAnnualReturnYear10

4.62%

Class B

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

5.00%

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

1.00%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.77%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

680

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

857

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

1,159

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

1,913

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

180

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

557

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

959

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

1,913

1 Year

rr_AverageAnnualReturnYear01

(6.84%)

5 Years

rr_AverageAnnualReturnYear05

6.08%

10 Years

rr_AverageAnnualReturnYear10

5.83%

Class C

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

1.00%

^[2]

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

1.00%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.77%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

280

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

557

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

959

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

2,084

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

180

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

557

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

959

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

2,084

1 Year

rr_AverageAnnualReturnYear01

(3.19%)

5 Years

rr_AverageAnnualReturnYear05

6.40%

10 Years

rr_AverageAnnualReturnYear10

5.71%

Class F

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

none

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.10%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.87%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

89

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

278

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

482

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

1,073

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

89

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

278

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

482

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

1,073

1 Year

rr_AverageAnnualReturnYear01

(1.41%)

Life of Class

rr_AverageAnnualReturnSinceInception

2.04%

Inception Date for Performance

rr_AverageAnnualReturnInceptionDate

Sep. 28, 2007

Class I

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

none

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

none

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

0.77%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

79

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

246

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

428

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

954

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

79

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

246

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

428

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

954

1 Year

rr_AverageAnnualReturnYear01

(1.30%)

5 Years

rr_AverageAnnualReturnYear05

7.46%

10 Years

rr_AverageAnnualReturnYear10

6.75%

Class P

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

none

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.45%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.22%

^[3]

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

124

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

387

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

670

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

1,477

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

124

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

387

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

670

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

1,477

1 Year

rr_AverageAnnualReturnYear01

(1.73%)

5 Years

rr_AverageAnnualReturnYear05

6.99%

10 Years

rr_AverageAnnualReturnYear10

6.27%

Class R2

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

none

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.60%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.37%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

139

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

434

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

750

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

1,646

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

139

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

434

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

750

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

1,646

1 Year

rr_AverageAnnualReturnYear01

(1.91%)

Life of Class

rr_AverageAnnualReturnSinceInception

1.52%

Inception Date for Performance

rr_AverageAnnualReturnInceptionDate

Sep. 28, 2007

Class R3

Risk/Return:

rr_RiskReturnAbstract

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice

none

Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)

rr_MaximumDeferredSalesChargeOverOther

none

Management Fees

rr_ManagementFeesOverAssets

0.51%

Distribution and Service (12b-1) Fees

rr_DistributionAndService12b1FeesOverAssets

0.50%

Other Expenses

rr_OtherExpensesOverAssets

0.26%

Total Annual Fund Operating Expenses

rr_ExpensesOverAssets

1.27%

Expense Example, with Redemption, 1 Year

rr_ExpenseExampleYear01

129

Expense Example, with Redemption, 3 Years

rr_ExpenseExampleYear03

403

Expense Example, with Redemption, 5 Years

rr_ExpenseExampleYear05

697

Expense Example, with Redemption, 10 Years

rr_ExpenseExampleYear10

1,534

Expense Example, No Redemption, 1 Year

rr_ExpenseExampleNoRedemptionYear01

129

Expense Example, No Redemption, 3 Years

rr_ExpenseExampleNoRedemptionYear03

403

Expense Example, No Redemption, 5 Years

rr_ExpenseExampleNoRedemptionYear05

697

Expense Example, No Redemption, 10 Years

rr_ExpenseExampleNoRedemptionYear10

$ 1,534

1 Year

rr_AverageAnnualReturnYear01

(1.81%)

Life of Class

rr_AverageAnnualReturnSinceInception

1.63%

Inception Date for Performance

rr_AverageAnnualReturnInceptionDate

Sep. 28, 2007

[1] A contingent deferred sales charge ("CDSC") of 1.00% may be assessed on certain Class A shares purchased or acquired without a sales charge if they are redeemed before the first day of the month of the one-year anniversary of the purchase.

[2] A CDSC of 1.00% may be assessed on Class C shares if they are redeemed before the first anniversary of their purchase.

[3] This amount has been updated from fiscal year amounts to reflect current fees and expenses.

XML

LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/

LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/

INVESTMENT OBJECTIVE

The Fund’s investment objective is long-term growth of capital through a diversified and actively managed portfolio consisting of developing growth companies, many of which are traded over the counter.

FEES AND EXPENSES

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and certain members of your family invest, or agree to invest in the future, at least $50,000 in the Lord Abbett Family of Funds. More information about these and other discounts is available from your financial professional and in “Sales Charge Reductions and Waivers” on page 24 of the prospectus and “Purchases, Redemptions, Pricing, and Payments to Dealers” on page 8-1 of the statement of additional information (“SAI”).

Shareholder Fees (Fees paid directly from your investment)

Shareholder Fees LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/	Class A		Class B	Class C		Class F	Class I	Class P	Class R2	Class R3
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)	5.75%		none	none		none	none	none	none	none
Maximum Deferred Sales Charge (Load) (as a percentage of offering price or redemption proceeds, whichever is lower)	none	[1]	5.00%	1.00%	[2]	none	none	none	none	none

Annual Fund Operating Expenses (Expenses that you pay each year as a percentage of the value of your investment)

Operating Expenses LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/	Class A	Class B	Class C	Class F	Class I	Class P		Class R2	Class R3
Management Fees	0.51%	0.51%	0.51%	0.51%	0.51%	0.51%		0.51%	0.51%
Distribution and Service (12b-1) Fees	0.35%	1.00%	1.00%	0.10%	none	0.45%		0.60%	0.50%
Other Expenses	0.26%	0.26%	0.26%	0.26%	0.26%	0.26%		0.26%	0.26%
Total Annual Fund Operating Expenses	1.12%	1.77%	1.77%	0.87%	0.77%	1.22%	[1]	1.37%	1.27%

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund at the maximum sales charge, if any, for the time periods indicated and then redeem all of your shares at the end of those periods. The example also assumes that your investment has a 5% return each year, that dividends and distributions are reinvested, and that the Fund’s operating expenses remain the same. The example assumes a deduction of the applicable contingent deferred sales charge (“CDSC”) for the one-year, three-year, and five- year periods for Class B shares and for the one-year period for Class C shares. Class B shares automatically convert to Class A shares after approximately eight years. The expense example for Class B shares for the ten-year period reflects the conversion to Class A shares. The first example assumes that you redeem all of your shares at the end of the periods. Although your actual costs may be higher or lower, based on these assumptions, your costs (including any applicable CDSC) would be as shown below. The second example assumes that you do not redeem and instead keep your shares.

If Shares Are Redeemed

Expense Example LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/ (USD $)	1 Year	3 Years	5 Years	10 Years
Class A	683	911	1,156	1,860
Class B	680	857	1,159	1,913
Class C	280	557	959	2,084
Class F	89	278	482	1,073
Class I	79	246	428	954
Class P	124	387	670	1,477
Class R2	139	434	750	1,646
Class R3	129	403	697	1,534

If Shares Are Not Redeemed

Expense Example No Redemption LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/ (USD $)	1 Year	3 Years	5 Years	10 Years
Class A	683	911	1,156	1,860
Class B	180	557	959	1,913
Class C	180	557	959	2,084
Class F	89	278	482	1,073
Class I	79	246	428	954
Class P	124	387	670	1,477
Class R2	139	434	750	1,646
Class R3	129	403	697	1,534

Portfolio Turnover.

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in the annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 195.11% of the average value of its portfolio.

PRINCIPAL INVESTMENT STRATEGIES

To pursue its objective, the Fund invests primarily in the common stocks of companies demonstrating above-average, long-term growth potential. The Fund seeks to identify companies that it believes are strongly positioned in the developing growth phase, which the Fund defines as the period of swift development after a company’s start-up phase when growth occurs at a rate rarely equaled by established companies in their mature years.

Under normal market conditions, the Fund invests at least 65% of its net assets in equity securities of small companies. In selecting investments, the Fund may invest in U.S. and foreign (which may include emerging market) companies. Foreign companies may be traded on U.S. or non-U.S. securities exchanges, may be denominated in the U.S. dollar or other currencies, and may include American Depositary Receipts (“ADRs”). Although the Fund is diversified across many industries and sectors, its assets may, from time to time, be overweighted or underweighted to certain industries and sectors relative to its benchmark index. The Fund’s principal investments include the following types of securities and other financial instruments:

• Equity securities usually include common stocks, but also may include preferred stocks, convertible securities, and equity interests in trusts, partnerships, and limited liability companies. The Fund also may invest in securities that are tied to the price of stock, including rights and convertible debt securities.

• Small companies having a market capitalization at the time of purchase that falls within the market capitalization range of companies in the Russell 2000® Index.

• Growth companies that the Fund believes exhibit faster-than-average gains in earnings and have the potential to continue profit growth at a high level.

The Fund may engage in active and frequent trading of its portfolio securities.

The Fund generally will sell a security when the Fund believes the security is less likely to benefit from the current market and economic environment, shows signs of deteriorating fundamentals, or has reached its valuation target, among other reasons. The Fund seeks to remain fully invested in accordance with its investment objective; however, in response to adverse economic, market or other unfavorable conditions, the Fund may invest its assets in a temporary defensive manner.

PRINCIPAL RISKS

As with any investment in a mutual fund, investing in the Fund involves risk, including the risk that you may receive little or no return on your investment. When you redeem your shares, they may be worth more or less than what you paid for them, which means that you may lose a portion or all of the money you invested in the Fund. The principal risks of investing in the Fund, which could adversely affect its performance, include:

• Portfolio Management Risk: If the strategies used and securities selected by the Fund’s portfolio management fail to produce the intended result, the Fund may suffer losses or underperform other funds with the same investment objective or strategies, even in a rising market.

• Equity Risk: Common stocks and other equity securities, as well as equity-like securities such as convertible bonds, may experience significant volatility. Such securities may fall sharply in response to adverse events affecting overall markets, a particular industry or sector, or an individual company’s financial condition.

• Small Company Risk: The Fund invests primarily in equity securities of small companies, which typically involve greater investment risks than larger companies. Small companies may have limited management experience or depth, limited ability to generate or borrow capital needed for growth, and limited products or services or operate in markets that have not yet been established. Accordingly, small company securities tend to be more sensitive to changing economic conditions and tend to be more volatile and less liquid than equity securities of larger companies.

• Growth Investing Risk: The Fund uses a growth investing style, which may be out of favor or may not produce the best results over short or longer time periods. In addition, growth stocks tend to be more volatile than slower-growing value stocks.

• Foreign Company Risk: The Fund’s investments in foreign (including emerging market) companies and in U.S. companies with economic ties to foreign markets generally involve special risks that can increase the likelihood that the Fund will lose money. For example, as compared with companies organized and operated in the U.S., these companies may be more vulnerable to economic, political, and social instability and subject to less government supervision, inadequate regulatory and accounting standards, and foreign taxes. In addition, the securities of foreign companies also may be subject to inadequate exchange control regulations, higher transaction and other costs, reduced liquidity, and delays in settlement to the extent they are traded on non-U.S. exchanges or markets. Emerging market securities generally are more volatile than other foreign securities, and are subject to greater liquidity, regulatory, and political risks.

• Industry/Sector Risk: To the extent the Fund overweights a single market sector or industry relative to its benchmark index, it can accumulate relatively large positions in a single issuer, industry, or sector. As a result, the Fund’s performance may be tied more directly to the success or failure of a relatively smaller or less diversified group of portfolio holdings.

• High Portfolio Turnover Risk: High portfolio turnover (more than 100%) may result in increased brokerage fees or other transaction costs, reduced investment performance, and higher taxes.

An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. For more information on the principal risks of the Fund, please see the “More Information About the Fund – Principal Risks” section in the prospectus.

PERFORMANCE

The bar chart and table below provide some indication of the risks of investing in the Fund by illustrating the variability of the Fund’s returns. Each assumes reinvestment of dividends and distributions. The Fund’s past performance, before and after taxes, is not necessarily an indication of how the Fund will perform in the future.

The bar chart shows changes in the performance of the Fund's ClassA shares from calendar year to calendar year. This chart does not reflect the sales charge applicable to ClassA shares. If the sales charge were reflected, returns would be lower. Performance for the Fund's other share classes will vary due to the different expenses each class bears. Updated performance information is available at www.lordabbett.com or by calling 888-522-2388.

Bar Chart (per calendar year) - Class A Shares*

* The total return for the Fund’s Class A shares for the nine-month period from January 1, 2012 to September 30, 2012 was +15.22%.

Best Quarter 2nd Q ‘09 +24.37% Worst Quarter 4th Q ’08 -28.60%

Average Annual Total Returns (for the periods ended December 31, 2011)

The table below shows how the Fund’s average annual total returns compare to the returns of a securities index. The Fund’s average annual total returns include applicable sales charges.

The after-tax returns of Class A shares included in the table below are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. In some cases, the return after taxes on distributions and sale of Fund shares may exceed the return before taxes due to a tax benefit resulting from realized losses on a sale of Fund shares at the end of the period that is used to offset other gains. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their Fund shares through tax- deferred arrangements such as 401(k) plans or Individual Retirement Accounts (“IRAs”). After-tax returns for other share classes are not shown in the table and will vary from those shown for Class A shares.

Average Annual Returns LORD ABBETT DEVELOPING GROWTH FUND INC /NEW/	1 Year	5 Years	10 Years	Life of Class	Inception Date for Performance
Class A	(7.32%)	5.83%	5.73%
Class A After Taxes on Distributions	(8.18%)	4.97%	4.86%
Class A After Taxes on Distributions and Sale of Fund Shares	(3.65%)	4.70%	4.62%
Class B	(6.84%)	6.08%	5.83%
Class C	(3.19%)	6.40%	5.71%
Class F	(1.41%)			2.04%	Sep. 28, 2007
Class I	(1.30%)	7.46%	6.75%
Class P	(1.73%)	6.99%	6.27%
Class R2	(1.91%)			1.52%	Sep. 28, 2007
Class R3	(1.81%)			1.63%	Sep. 28, 2007
Russell 2000® Growth Index (reflects no deduction for fees, expenses, or taxes)	(2.91%)	2.09%	4.48%	0.33%	Sep. 28, 2007

[1] A contingent deferred sales charge ("CDSC") of 1.00% may be assessed on certain Class A shares purchased or acquired without a sales charge if they are redeemed before the first day of the month of the one-year anniversary of the purchase.

[2] A CDSC of 1.00% may be assessed on Class C shares if they are redeemed before the first anniversary of their purchase.

[1] This amount has been updated from fiscal year amounts to reflect current fees and expenses.

XML

Document and Entity Information	0 Months Ended
	Jul. 31, 2012
Risk/Return:
Document Type	485BPOS
Document Period End Date	Jul. 31, 2012
Registrant Name	LORD ABBETT DEVELOPING GROWTH FUND, INC.
Central Index Key	0000276914
Amendment Flag	false
Document Creation Date	Nov. 27, 2012
Document Effective Date	Dec. 01, 2012
Prospectus Date	Dec. 01, 2012

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Prospectus Date	rr_ProspectusDate	Dec. 01, 2012