Item 1.01 Entry into a Material Definitive Agreement.

On November 17, 2009, the registrant, Cubist Pharmaceuticals, Inc. ("Cubist") and ACS Dobfar, SpA ("ACS") entered into a Fifth Amendment to the September 30, 2001 Manufacturing and Supply Agreement under which ACS supplies Cubist with Active Pharmaceutical Ingredient ("API") for CUBICINŽ (daptomycin for injection). Under the Fifth Amendment, Cubist and ACS have agreed to: (a) an implementation plan and sharing of costs for certain improvements to ACS's manufacturing facility relating to the manufacture of CUBICIN API, (b) a new pricing schedule that can be updated further in the event of future facility or process improvements, and (c) a new minimum order requirement structure based on a percentage of Cubist's CUBICIN API requirements rather than an absolute annual minimum.

The foregoing description of the Fifth Amendment contained in this Item 1.01 does not purport to be complete and is qualified in its entirety by reference to the complete text of the Fifth Amendment, which will be filed, with confidential terms redacted, with the Securities and Exchange Commission as an exhibit to the registrant's Annual Report on Form 10-K for the year ending on December 31, 2009.

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