Xenogen Corporation Announces Initial Public Offering

Alameda, California, July 16, 2004 - Xenogen Corporation (Nasdaq: XGEN) announced today the initial public offering of 4,200,000 shares of common stock at a price of $7.00 per share. All of the shares are being sold by the Company. The Company has granted the underwriters an option to purchase an additional 630,000 shares to cover overallotments, if any. Xenogen’ s common stock will be quoted on the Nasdaq National Market under the symbol “XGEN.”

Thomas Weisel Partners LLC is serving as the book running manager for the offering with CIBC World Markets serving as a co-lead manager and JMP Securities LLC as a comanager. A copy of the final prospectus relating to the offering may be obtained from Thomas Weisel Partners LLC at One Montgomery Street, Suite 3700, San Francisco, California 94104.

Founded in 1995, Xenogen is a leader in the field of real-time in vivo imaging and sells an integrated system of instruments and equipment, software, and reagents to academic and biopharmaceutical researchers to improve the productivity and efficiency of the drug discovery and development process.