Island vertebrate consumers of marine crustaceans and fish are particularly susceptible to seasonal fluctuations of oceanographic conditions. The endemic fish-eating myotis (Myotis vivesi) is restricted to a few insular ecosystems in the Gulf of California, where it feeds on both marine and terrestrial prey; yet it is unknown whether its diet covaries with marine primary productivity. We determined the relative contribution of marine and terrestrial food sources to the diet of the fish-eating myotis to test the hypothesis that seasonal changes in oceanic primary productivity would dictate general feeding patterns of this bat species. We predicted that marine food would predominate in the diet of the bat during winter followed by an increase in importance of terrestrial food sources as summer approached. We measured amount of feces deposited in the day roost per bat and extrapolated this value to estimate amount of feces deposited in 1 year by the fish-eating myotis population in Partida Norte Island. Carbon and nitrogen stable isotope analysis of bat blood and examination of food remains in feces showed that marine food was the main source of nutrients for fish-eating myotis throughout the year. Importance of terrestrial food was marginal during most of the year, with a modest increase in summer. Estimated annual feces production was up to 1,518 kg. Because of its predominantly marine feeding habits, fish-eating myotis transfer allochthonous nutrients to islands where they roost, subsidizing the nutrient and energy economy of the islands. © 2013 American Society of Mammalogists.