-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 LWqlglDnSRs4LfD9L7VqL8uJFW/IctadyfWWBH79JeHGawaR0NTDoPTW1cZPOfQp
 dHl3jT82qJkm15QP1D5vOQ==

<SEC-DOCUMENT>0000107832-05-000147.txt : 20050719
<SEC-HEADER>0000107832-05-000147.hdr.sgml : 20050719
<ACCEPTANCE-DATETIME>20050719120823
ACCESSION NUMBER:		0000107832-05-000147
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050718
FILED AS OF DATE:		20050719
DATE AS OF CHANGE:		20050719

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			ALLIANT ENERGY CORP
		CENTRAL INDEX KEY:			0000352541
		STANDARD INDUSTRIAL CLASSIFICATION:	ELECTRIC & OTHER SERVICES COMBINED [4931]
		IRS NUMBER:				391380265
		STATE OF INCORPORATION:			WI
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		4902 NORTH BILTMORE LANE
		STREET 2:		PO BOX 77007
		CITY:			MADISON
		STATE:			WI
		ZIP:			53707-1007
		BUSINESS PHONE:		608-458-3314

	MAIL ADDRESS:	
		STREET 1:		4902 NORTH BILTMORE LANE
		STREET 2:		PO BOX 77007
		CITY:			MADISON
		STATE:			WI
		ZIP:			53707-1007

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	INTERSTATE ENERGY CORP
		DATE OF NAME CHANGE:	19980427

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	WPL HOLDINGS INC
		DATE OF NAME CHANGE:	19920703

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			HARVEY WILLIAM D
		CENTRAL INDEX KEY:			0001209101

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-09894
		FILM NUMBER:		05960985

	MAIL ADDRESS:	
		STREET 1:		5 SHADE TREE COURT
		CITY:			MADISON
		STATE:			WI
		ZIP:			53717
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>c0001209101d20050719f4f.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-07-18</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000352541</issuerCik>
        <issuerName>ALLIANT ENERGY CORP</issuerName>
        <issuerTradingSymbol>LNT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001209101</rptOwnerCik>
            <rptOwnerName>HARVEY WILLIAM D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>PO BOX 2568</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MADISON</rptOwnerCity>
            <rptOwnerState>WI</rptOwnerState>
            <rptOwnerZipCode>53701</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>PRESIDENT &amp; CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>DEFERRED COMMON STOCK</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-07-18</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6.6709</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>28.25</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>1988-08-08</value>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>1988-08-08</value>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>COMMON</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>6.6709</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>36986.4518</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Units are to be settled upon reporting person's retirement.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>F. J. Buri as POA for</signatureName>
        <signatureDate>2005-07-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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