We analyzed the stomach contents of 52 blue marlins caught between October 2002 and October 2004 by the sport-fishing fleet of Mazatlan, Sinaloa, in the gulf of California, Mexico. Blue marlin feed on 15 food items. According to the index of relative importance (IRI), the most important prey were the frigate or bullet mackerel Auxis spp. (52%) and jumbo squid Dosidicus gigas (30%).