Through the analysis of 458 barn owl (Tyto alba) pellets collected between 1981 and 1995 on 2 islands in the northern Gulf of California, Mexico, we found significant differences in the main components of the diet of barn owls on these islands, where introduced rodents, bats, and seabirds occur. On Rasa Island, the most important prey was Rattus rattus, followed by Myotis vivesi, Oceanodroma microsoma, Mus musculus, and 4 bird species. On Partida Island, the main prey was M. vivesi, followed by O. microsoma, R. rattus, and 3 bird species. The mean number of prey per pellet was higher on Partida Island (1.08) than on Rasa Island (0.91), but the mean biomass per pellet was higher on Rasa Island (75.8 g) than on Partida Island (25.8 g). The storm-petrels O. microsoma and O. melania were taken by barn owls in proportion to their abundance on both islands. Conversely, barn owls on Rasa Island ate more R. rattus than M. musculus, possibly because of the larger size of the former. Overall, our data suggest that barn owls in the Gulf of California behave as flexible predators that take prey according to availability and profitability.