Activision announced yesterday that it would buy game developer Treyarch Invention for up to almost $20 million in stock in a move to bolster its in-house game development.

Santa Monica based Treyarch, founded in 1996, has developed a number of games for Activision, including "Tony Hawk's Pro Skater 2'' and "Spider-Man'' for Sega Corp.'s Dreamcast game system. The Tony Hawk skateboarding games have been consistently large sellers for Activision. Activision would take on Treyarch's 140 employees, and the company said it does not expect the acquisition would affect revenue or earnings-per-share forecasts for fiscal 2002 or 2003.

Activision shares were up $4.00 a share or more than 15 percent at $29.75 on Nasdaq in afternoon trade. Year-to-date, Activision shares are up 95 percent, making it one of the strongest performers in the sector.