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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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<SEC-DOCUMENT>0001181431-03-016799.txt : 20030801
<SEC-HEADER>0001181431-03-016799.hdr.sgml : 20030801
<ACCEPTANCE-DATETIME>20030801172218
ACCESSION NUMBER:		0001181431-03-016799
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20030731
FILED AS OF DATE:		20030801

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			KOSAN BIOSCIENCES INC
		CENTRAL INDEX KEY:			0001110206
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-COMMERCIAL PHYSICAL & BIOLOGICAL RESEARCH [8731]
		IRS NUMBER:				943217016
		STATE OF INCORPORATION:			CA
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		3832 BAY CENTER PLACE
		CITY:			HAYWARD
		STATE:			CA
		ZIP:			94545
		BUSINESS PHONE:		5107328400

	MAIL ADDRESS:	
		STREET 1:		3832 BAY CENTER PLACE
		CITY:			HAYWARD
		STATE:			CA
		ZIP:			94545

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			OSTRACH MICHAEL S
		CENTRAL INDEX KEY:			0001226066

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-31633
		FILM NUMBER:		03818849

	BUSINESS ADDRESS:	
		STREET 1:		C/O KOSAN BIOSCIENCES
		STREET 2:		3832 BAY CENTER PALCE
		CITY:			HAYWARD
		STATE:			CA
		ZIP:			94545
		BUSINESS PHONE:		5107328400
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>rrd14751.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?><ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-07-31</periodOfReport><notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0001110206</issuerCik>
        <issuerName>KOSAN BIOSCIENCES INC</issuerName>
        <issuerTradingSymbol>KOSN</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001226066</rptOwnerCik>
            <rptOwnerName>OSTRACH MICHAEL S</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>3832 BAY CENTER PLACE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>HAYWARD</rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode>94545</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>President and COO</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-07-31</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F1"/></transactionCoding><transactionAmounts><transactionShares><value>10000</value></transactionShares><transactionPricePerShare><value>7.906</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>208450</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwne
rship><natureOfOwnership><value>see footnote</value><footnoteId id="F2"/></natureOfOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><footnotes><footnote id="F1">Sale pursuant to rule 10(b)5-1</footnote><footnote id="F2">Shares are held by Michael Ostrach and Karen Ostrach, Trustees FBO The Michael &amp; Karen Ostrach Family Trust</footnote></footnotes><ownerSignature><signatureName>Susan Kanaya, Attorney-in-Fact</signatureName><signatureDate>2003-08-01</signatureDate></ownerSignature></ownershipDocument>



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