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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 BETHHnKfN2VWjFVIgElzyzTgAGw90y3xd51TB8XHKtlO/xgvEySQX4qsdI+Quo9r
 +sgIHqDdpjLV6vlhCTujaA==

<SEC-DOCUMENT>0001335038-05-000001.txt : 20050812
<SEC-HEADER>0001335038-05-000001.hdr.sgml : 20050812
<ACCEPTANCE-DATETIME>20050812092919
ACCESSION NUMBER:		0001335038-05-000001
CONFORMED SUBMISSION TYPE:	3
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050802
FILED AS OF DATE:		20050812
DATE AS OF CHANGE:		20050812

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			CUBIST PHARMACEUTICALS INC
		CENTRAL INDEX KEY:			0000912183
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				223192085
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		65 HAYDEN AVENUE
		CITY:			LEXINGTON
		STATE:			MA
		ZIP:			02421
		BUSINESS PHONE:		781-860-8660

	MAIL ADDRESS:	
		STREET 1:		24 EMILY ST
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Fellows Lindon M
		CENTRAL INDEX KEY:			0001335038

	FILING VALUES:
		FORM TYPE:		3
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-21379
		FILM NUMBER:		051019181

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		978-352-6220

	MAIL ADDRESS:	
		STREET 1:		26 HOVEY'S POND DRIVE
		CITY:			BOXFORD
		STATE:			MA
		ZIP:			01921
</SEC-HEADER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2005-08-02</periodOfReport>

    <noSecuritiesOwned>1</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000912183</issuerCik>
        <issuerName>CUBIST PHARMACEUTICALS INC</issuerName>
        <issuerTradingSymbol>CBST</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001335038</rptOwnerCik>
            <rptOwnerName>Fellows Lindon M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>26 HOVEY'S POND DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>BOXFORD</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>01921</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Senior Vice President</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <footnotes></footnotes>

    <ownerSignature>
        <signatureName>Christopher D.T. Guiffre, Esq. - Attorney in fact</signatureName>
        <signatureDate>2005-08-12</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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