As filed with the U.S. Securities and Exchange Commission on March 13, 2008

Securities Act File No. 2-91556

Investment Company Act File No. 811-4052

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.
Post-Effective Amendment No. 51
and/or
REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940	x

Amendment No. 58

(Check appropriate box or boxes.)

Legg Mason Partners Money Market Trust*

(Exact Name of Registrant as Specified in Charter)

125 Broad Street, New York, New York 10004

(Address of Principal Executive Offices) (Zip Code)

Registrant’s Telephone Number, including Area Code (800) 451-2010

Robert I. Frenkel

Legg Mason Partners Money Market Trust

300 First Stamford Place

Stamford, Connecticut

(Name and Address of Agent for Service)

COPY TO:

Roger P. Joseph, Esq.

Bingham McCutchen LLP

150 Federal Street

Boston, Massachusetts 02110

Continuous

(Approximate Date of Proposed Offering)

It is proposed that this filing will become effective on April 1, 2008 pursuant to Rule 485(b) under the Securities Act of 1933, as amended.

* This filing relates solely to Class W shares of Western Asset Money Market Fund and Western Asset Government Money Market Fund.

Part A (the Prospectus) and Part B (the Statement of Additional Information) filed by the Registrant in Post-Effective Amendment No. 47 to the Registration Statement on Form N-1A under the Securities Act of 1933 (File No. 2-91556) and Amendment No. 54 to the Registration Statement on Form N-1A under the Investment Company Act of 1940 (File No. 811-4052) pursuant to Rule 485(a) on November 16, 2007 (Accession Number 0001193125-07-249370) are herein incorporated by reference. This filing is being made pursuant to Rule 485(b)(1)(iii) under the Securities Act of 1933 to extend the effectiveness of the Registration Statement until April 1, 2008.

Part C

Other Information

Item 23. Exhibits

(a)(1) Declaration of Trust dated October 2, 2006 is incorporated herein by reference to Post-Effective Amendment No. 44 to the Registrant’s Registration Statement on Form N-1A as filed with the SEC on April 16, 2007 (“Post-Effective Amendment No. 44”).

(2) Designation of Series of Shares of Beneficial Interests in the Trust, effective as of February 6, 2007 is incorporated herein by reference to Post-Effective Amendment No. 44.

(3) Amended and Restated Designation of Classes to be filed by amendment.

(b) By-laws dated October 4, 2006 are incorporated herein by reference to Post-Effective Amendment No. 44.

(c) Not Applicable

(d)(1) Management Agreement between the Registrant, on behalf of Citi California Tax Free Reserves, and Legg Mason Partners Fund Advisor, LLC (“LMPFA”) dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45 to the Registrant’s Registration Statement on Form N-1A as filed with the SEC on July 27, 2007 (“Post-Effective Amendment No. 45”).

(2) Management Agreement between the Registrant, on behalf of Citi Cash Reserves, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(3) Management Agreement between the Registrant, on behalf of Citi Connecticut Tax Free Reserves, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(4) Management Agreement between the Registrant, on behalf of Citi New York Tax Free Reserves, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(5) Management Agreement between the Registrant, on behalf of Citi Tax Free Reserves, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(6) Management Agreement between the Registrant, on behalf of Citi U.S. Treasury Reserves, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(7) Management Agreement between the Registrant, on behalf of Western Asset Money Market Fund (formerly Cash Portfolio), and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(8) Management Agreement between the Registrant, on behalf of Western Asset Government Money Market Fund (formerly Government Portfolio), and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(9) Management Agreement between the Registrant, on behalf of Western Asset California Municipal Money Market Fund, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(10) Management Agreement between the Registrant, on behalf of Western Asset Massachusetts Municipal Money Market Fund, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(11) Management Agreement between the Registrant, on behalf of Western Asset Municipal Money Market Fund, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(12) Management Agreement between the Registrant, on behalf of Western Asset New York Municipal Money Market Fund, and LMPFA dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(13) Subadvisory Agreement between LMPFA and Western Asset Management Company (“WAM”), with respect to Citi California Tax Free Reserves, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(14) Subadvisory Agreement between LMPFA and WAM, with respect to Citi Cash Reserves, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(15) Subadvisory Agreement between LMPFA and WAM, with respect to Citi Connecticut Tax Free Reserves, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(16) Subadvisory Agreement between LMPFA and WAM, with respect to Citi New York Tax Free Reserves, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(17) Subadvisory Agreement between LMPFA and WAM, with respect to Citi Tax Free Reserves, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(18) Subadvisory Agreement between LMPFA and WAM, with respect to Citi U.S. Treasury Reserves, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(19) Subadvisory Agreement between LMPFA and WAM, with respect to Western Asset Money Market Fund (formerly Cash Portfolio), dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(20) Subadvisory Agreement between LMPFA and WAM, with respect to Western Asset Government Money Market Fund (formerly Government Portfolio), dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(21) Subadvisory Agreement between LMPFA and WAM, with respect to Western Asset California Municipal Money Market Fund, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(22) Subadvisory Agreement between LMPFA and WAM, with respect to Western Asset Massachusetts Municipal Money Market Fund, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(23) Subadvisory Agreement between LMPFA and WAM, with respect to Western Asset Municipal Money Market Fund, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(24) Subadvisory Agreement between LMPFA and WAM, with respect to Western Asset New York Municipal Money Market Fund, dated April 13, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(e)(1) Distribution Agreement between the Registrant, on behalf of Citi California Tax Free Reserves, Citi Cash Reserves, Citi Connecticut Tax Free Reserves, Citi New York Tax Free Reserves, Citi Tax Free Reserves and Citi U.S. Treasury Reserves, and Legg Mason Investor Services, LLC (“LMIS”), as distributor is incorporated herein by reference to Post-Effective Amendment No. 38.

(2) Distribution Agreement dated December 1, 2005, between Legg Mason Partners Municipal Funds, on behalf of Western Asset California Municipal Money Market Fund, Western Asset Massachusetts Municipal Money Market Fund and Western Asset New York Municipal Money Market Fund, and LMIS, as distributor, is incorporated herein by reference to the Post-Effective Amendment No. 54 to the Legg Mason Partners Municipal Funds Registration Statement on Form N-1A as filed with the SEC on July 31, 2006.

(3) Distribution Agreement dated December 1, 2005, between Smith Barney Municipal Money Market Fund, Inc., on behalf of Western Asset Municipal Money Market Fund, and LMIS, as distributor, is incorporated herein by reference to the Post-Effective Amendment No. 38 to the Smith Barney Municipal Money Market Fund, Inc. Registration Statement on Form N-1A as filed with the SEC on July 31, 2006.

(4) Distribution Agreement, dated December 1, 2005, between Smith Barney Money Funds, Inc. and LMIS is incorporated by reference to the Smith Barney Money Funds, Inc. Registration Statement on Form N-14 as filed with the SEC on July 21, 2006.

(5) Letter Agreement amending the Distribution Agreements with LMIS dated April 5, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(f)(1) Amended and Restated General Retirement Plan relating to certain funds is incorporated herein by reference to Post-Effective Amendment No. 37 to the Registrant’s Registration Statement on Form N-1A as filed with the SEC on October 31, 2005 (“Post-Effective Amendment No. 37”).

(2) Legg Mason Investment Series (f/k/a Smith Barney Investment Series) Amended and Restated Trustees Retirement Plan dated as of January 1, 2005, is incorporated herein by reference to Post-Effective Amendment No. 78 to the Legg Mason Partners Income Trust Registration Statement on Form N-1A as filed with the SEC on January 8, 2007.

(3) Amendment to the General Retirement Plan and the Legg Mason Partners Investment Series Amended and Restated Trustees Retirement is incorporated herein by reference to Post-Effective Amendment No. 40.

(4) Amended and Restated Emeritus Retirement Plan relating to certain funds established effective as of January 1, 2007, is incorporated herein by reference to Post-Effective Amendment No. 78 to the Legg Mason Partners Income Trust Registration Statement on Form N-1A as filed with the SEC on January 8, 2007.

(5) Emeritus Retirement Plan relating to certain funds established effective as of January 1, 2007, is incorporated herein by reference to Post-Effective Amendment No. 78 to the Legg Mason Partners Income Trust Registration Statement on Form N-1A as filed with the SEC on January 8, 2007.

(g)(1) Custodian Services Agreement with State Street Bank and Trust Company (“State Street”) dated January 1, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(2) Letter Agreement amending the Custodian Services Agreement with State Street dated April 9, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(h)(1) Form of Transfer Agency Agreement with PFPC Inc., as transfer agent is incorporated herein by reference to Post-Effective Amendment No. 38.

(2) Form of Transfer Agency Agreement between the Registrant and Boston Financial Data Services, Inc., as transfer agent is incorporated herein by reference to Post-Effective Amendment No. 38.

(3) Service Mark Licensing Agreement between Citigroup, Inc. and the Registrant is incorporated herein by reference to Post-Effective Amendment No. 38.

(4) Letter Agreement dated April 9, 2007, amending the Transfer Agency and Services Agreement with PFPC Inc. is incorporated herein by reference to Post-Effective Amendment No. 45.

(i)(1) Opinion and Consent of Counsel is incorporated herein by reference to Post-Effective Amendment No. 30.

(2) Opinion of Counsel regarding the legality of shares being registered is incorporated herein by reference to Post-Effective Amendment No. 44.

(3) Opinion of Counsel regarding the legality of shares being registered to be filed by amendment.

(j)(1) Not applicable.

(2) Power of Attorney dated February 6, 2007 is incorporated herein by reference to Post-Effective Amendment No. 44.

(k) Not Applicable.

(l) Not Applicable.

(m) Shareholder Services and Distribution Plan, pursuant to Rule 12b-1, of the Registrant, on behalf of Citi California Tax Free Reserves, Citi Cash Reserves, Citi Connecticut Tax Free Reserves, Citi New York Tax Free Reserves, Citi Tax Free Reserves, Citi U.S. Treasury Reserves, Western Asset Money Market Fund, Western Asset Government Money Market Fund, Western Asset California Municipal Money Market Fund, Western Asset Massachusetts Municipal Money Market Fund, Western Asset New York Municipal Money Market Fund, Western Asset Municipal Money Market Fund, Legg Mason Partners Exchange Reserves Fund and Legg Mason Partners New York Municipal Money Market Fund dated February 6, 2007 is incorporated herein by reference to Post-Effective Amendment No. 48 to the Registrant’s Registration Statement on Form N-1A as filed with the SEC on December 7, 2007.

(n) Rule 18f-3(d) Multiple Class Plan of the Registrant dated February 6, 2007 is incorporated herein by reference to Post-Effective Amendment No. 45.

(o) Not Applicable

(p)(1) Code of Ethics of Citigroup Asset Management—North America and Certain Registered Investment Companies, as amended September 13, 2005 (adopted by LMPFA), is incorporated herein by reference to Post-Effective Amendment No. 38.

(2) Code of Ethics of LMIS dated December 1, 2005 is incorporated herein by reference to Post-Effective Amendment No. 38.

(3) Code of Ethics of WAM dated as of February, 2005, is incorporated herein by reference to Post-Effective Amendment No. 39.

Item 24. Persons Controlled by or under Common Control with Registrant

Not Applicable.

Item 25. Indemnification

Provisions relating to indemnification of the Registrant’s Trustees and employees are included in Article IX of the Registrant’s Declaration of Trust, which is incorporated herein by reference.

Reference is hereby made to paragraph 10 of the Distribution Agreement between the Registrant and LMIS.

The Trustees and officers of the Registrant and the personnel of the Registrant’s manager are insured under an errors and omissions liability insurance policy. The Registrant and its officers are also insured under the fidelity bond required by Rule 17g-1 under the Investment Company Act of 1940, as amended.

Item 26. Business and Other Connections of Investment Adviser

Investment Adviser — Legg Mason Partners Fund Advisor, LLC (“LMPFA”)

LMPFA was formed in 2006 under the laws of the State of Delaware as a limited liability company. LMPFA is a direct wholly-owned subsidiary of Legg Mason, Inc. (“Legg Mason”).

LMPFA is registered as an investment adviser under the Investment Advisers Act of 1940, as amended. The list required by this Item 26 of officers and directors of LMPFA together with information as to any other business, profession, vocation or employment of a substantial nature engaged in by such officers and directors during the past two years, is incorporated by reference to Schedules A and D of Form ADV filed by LMPFA pursuant to the Investment Advisers Act of 1940, as amended (SEC File No. 801-66785).

Subadviser — Western Asset Management Company (“WAM”) is an investment adviser registered with the SEC under the Advisers Act. The following is a list of other substantial business activities in which directors, officers or partners of WAM have been engaged as director, officer, employee, partner, or trustee.

Peter L. Bain	Director, WAM
	Director, LMFM
	Manager, Brandywine
	Senior Executive Vice President, Legg Mason, Inc.
	Director, Nova Scotia
	Vice President and Director, BMML
	Director, LMCM
	Director, Bartlett
	Director, Berkshire
	Director, LM Funding
	Director, LM Properties
	Director, LMRG
	Director, LM Tower
	Director, PCM I
	Director, PCM II
	Manager, Royce
	Director, Western Asset Management Company Limited

James W. Hirschmann III	Director, WAM
	Director, Western Asset Management Company Limited
D. Daniel Fleet	President and CEO, WAM
Gavin L. James	Director of Global Client Services, WAM
	Senior Executive Officer, Western Asset Management Company Limited
Gregory McShea	General Counsel and Secretary, WAM
	General Counsel and Secretary, Western Asset Management
	Company Limited

WAM is located at 385 East Colorado Boulevard, Pasadena, CA 91101.

Item 27. Principal Underwriters

(a) Legg Mason Investor Services, LLC (“LMIS”), a distributor of the Registrant is also a distributor of the following funds: Legg Mason Partners Premium Money Market Trust, Legg Mason Partners Institutional Trust, Legg Mason Partners Money Market Trust, Western Asset High Income Opportunity Fund Inc., Western Asset Intermediate Muni Fund, Inc., Legg Mason Partners Equity Trust, LMP Real Estate Income Fund Inc., Western Asset Managed High Income Portfolio Inc., Western Asset Managed Municipals Portfolio Inc., Western Asset Municipal High Income Fund Inc., LMP Corporate Loan Fund Inc., Western Asset Zenix Income Fund Inc., Barrett Opportunity Fund, Inc., Western Asset 2008 Worldwide Government Term Trust Inc., Western Asset High Income Fund Inc., Western Asset High Income Fund II Inc., Western Asset Emerging Markets Income Fund Inc., Western Asset Emerging Markets Income Fund II Inc., Western Asset Emerging Markets Floating Rate Fund Inc., Western Asset Global High Income Fund Inc., Western Asset Emerging Markets Debt Fund Inc., LMP Capital and Income Fund Inc., Western Asset Inflation Management Fund Inc., Western Asset Variable Rate Strategic Fund Inc., Western Asset Global Partners Income Fund Inc., Western Asset Municipal Partners Fund Inc., Western Asset Municipal Partners Fund II Inc., Legg Mason Partners Variable Income Trust, Legg Mason Partners Income Trust, Legg Mason Cash Reserve Trust, Inc., Legg Mason Charles Street Trust, Inc., Legg Mason Global Trust, Inc., Legg Mason Growth Trust, Inc., Legg Mason Income Trust, Inc., Legg Mason Investment Trust, Inc., Legg Mason Investors Trust, Inc., Legg Mason Light Street Trust, Inc., Legg Mason Special Investment Trust, Inc., Legg Mason Tax Exempt Trust, Inc., Legg Mason Tax-Free Income Fund, Legg Mason Value Trust, Inc., Western Asset Funds, Inc. and Legg Mason Partners Variable Equity Trust.

LMIS is the placement agent for Institutional Enhanced Portfolio, Prime Cash Reserves Portfolio, U.S. Treasury Reserves Portfolio, Tax Free Reserves Portfolio and Liquid Reserves Portfolio.

(b) The information required by this Item 27 with respect to each director and officer of LMIS is listed below:

Mark R. Fetting – Managing Director

D. Stuart Bowers – Vice President

W. Talbot Daley – Vice President

Thomas J. Hirschmann – Vice President

Joseph M. Furey – General Counsel and Chief Compliance Officer

Ronald Holinsky – Counsel

Robert E. Patterson – Counsel

Theresa M. Silberzahn – Chief Financial Officer

Elisabeth F. Craig – AML Compliance Officer and Director of Continuing Education

All Addresses are 100 Light Street, Baltimore, Maryland 21202.

(c) Not applicable.

Item 28. Location of Accounts and Records

With respect to the Registrant:

(1) Legg Mason Partners Money Market Trust

125 Broad Street

New York, New York 10004

With respect to the Registrant’s Investment Manager:

(2) c/o Legg Mason Partners Fund Advisor, LLC

620 Eighth Avenue

New York, NY 10018

With respect to the Registrant’s Subadviser:

(3) c/o Western Asset Management Company

620 Eighth Avenue

New York, NY 10018

With respect to the Registrant’s Custodian:

(4) State Street Bank & Trust Company

One Lincoln Street

Boston, MA 02111

With respect to the Registrant’s Transfer Agents:

(5) PFPC Inc.

P.O. Box 9699

Providence, Rhode Island 02940-9699

(6) Boston Financial Data Services, Inc.

2 Heritage Drive

North Quincy, MA 02171

With respect to the Registrant’s Distributor:

(7) Legg Mason Investor Services, LLC

100 Light Street

Baltimore, MD 21202

Item 29. Management Services

Not applicable.

Item 30. Undertakings

Not applicable.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended (the “Securities Act”), and the Investment Company Act of 1940, as amended, the Registrant, LEGG MASON PARTNERS MONEY MARKET TRUST, certifies that it meets all requirements for effectiveness of this Post-Effective Amendment to the Registration Statement pursuant to Rule 485(b) under the Securities Act and has duly caused this Post-Effective Amendment to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Stamford, State of Connecticut on this 13th day of March, 2008.

LEGG MASON PARTNERS MONEY MARKET TRUST, on behalf of its series:

Western Asset Money Market Fund and

Western Asset Government Money Market Fund

By:	/s/ R. Jay Gerken
	R. Jay Gerken
	President and Principal Executive Officer

WITNESS our hands on the date set forth below.

Pursuant to the requirements of the Securities Act, this Post-Effective Amendment to the Registration Statement has been signed below by the following persons in the capacities indicated below on March 13, 2008.

Signature		Title
	/s/ R. Jay Gerken	President, Principal Executive Officer and
	R. Jay Gerken	Trustee
	/s/ Frances M. Guggino	Treasurer and Chief Financial Officer
	Frances M. Guggino
	/s/ Elliott J. Berv*	Trustee
	Elliott J. Berv
	/s/ A. Benton Cocanougher*	Trustee
	A. Benton Cocanougher
	/s/ Jane F. Dasher*	Trustee
	Jane F. Dasher
	/s/ Mark T. Finn*	Trustee
	Mark T. Finn
	/s/ Rainer Greeven*	Trustee
	Rainer Greeven
	/s/ Stephen R. Gross*	Trustee
	Stephen R. Gross
	/s/ Richard E. Hanson, Jr.*	Trustee
	Richard E. Hanson, Jr.
	/s/ Diana R. Harrington*	Trustee
	Diana R. Harrington

	/s/ Susan M. Heilbron*	Trustee
	Susan M. Heilbron
	/s/ Susan B. Kerley*	Trustee
	Susan B. Kerley
	/s/ Alan G. Merten*	Trustee
	Alan G. Merten
	/s/ R. Richardson Pettit*	Trustee
	R. Richardson Pettit
*By:	/s/ R. Jay Gerken
	R. Jay Gerken

* Attorney-in-Fact, pursuant to Power of Attorney.