Well miacrow, you did say you have a Mac, right? Try asking in the Civ3 - Macintosh forum, someone there might know. I'm guessing everyone else here uses PC's with some version of Windows, and I've never heard of Civ3 asking anyone for a serial number when running on Windows. I have Civ3 Complete (from Civilization Chronicles) as well as the individual disks, and I've never had to enter a serial. Is it asking you that for registration or to actually run the game?