Socorro Island, the largest and biologically most diverse of the four islands of the Revillagigedo Archipelago (Brattstrom 1990), confronts serious problems related to human activities. Eight endemic taxa of land birds now breed on Socorro: Yellow-crowned Night Heron, Nyctanassa violacea gravirostris; Socorro Red-tailed Hawk, Buteo jamaicensis socorroensis; Socorro Ground Dove, Columbina passerina socorroensis; Green Parakeet, Aratinga holochlora brevipes; Socorro Wren, Thryomanes sissonii; Socorro Mockingbird, Mimodes graysoni; Tropical Parula Warbler, Parula pitiayumi graysoni; and Socorro Towhee, Pipilo erythrophthalmus socorroensis (McLellan 1926, Jehl and Parkes 1982, Brattstrom 1990). Jehl and Parkes (1983) have proposed that domestic cats (Felis catus), introduced since 1957, are responsible for the extirpation of the endemic Socorro Dove, Zenaida graysoni, and reductions of other species, especially the Socorro Mockingbird. However, no quantitative data on predation by feral cats on Socorro Island have been previously reported in the literature. We present here an analysis of the diet of feral cats on Socorro Island.