Exhibit 32.2

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, Inc. (“Cubist”) on Form 10-Q for the period ending June 30, 2010, as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, David W.J. McGirr, Chief Financial Officer, 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.

July 30, 2010

/s/ David W.J. McGirr

David W.J. McGirr

Senior Vice President and

Chief Financial Officer