Cubist Pharmaceuticals (CBST) 10-Q 2004 Q1 Quarterly report

Exhibit 32.1

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350,
AS ADOPTED PURSUANT TO SECTION 906 OF
THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Cubist Pharmaceuticals (the “Company”) on Form 10-Q for the period ending March 31, 2004 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Michael W. Bonney, President and Chief Executive Officer of Cubist, certify, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that: (1) The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and (2) The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of Cubist.

May 7, 2004

/s/ Michael W. Bonney

Michael W. Bonney

President and Chief Executive Officer

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