This study documents the diet of Mexican free-tailed bats (Tadarida brasiliensis mexicana) at Carlsbad Cavern, Carlsbad Caverns National Park, Eddy County, New Mexico, and provides information on seasonal variation in food habits of this species throughout its summer residence. Diet was determined from 1,303 fecal samples. Eleven orders and 38 families of insects, unidentified insects, 2 orders of Arachnida (Araneae and Acari), bat hair, and mist net were consumed, with Lepidoptera (moths) and Coleoptera (beetles) occurring at greatest percentage volumes and percentage frequencies in the diet. Diet varied significantly throughout the season. Twenty-two food categories exhibited statistically significant variation among sampling sessions. Amounts of Coleoptera, Lepidoptera, Formicidae, Psyllidae, Hymenoptera, and Diptera consumed showed the greatest differences among sampling sessions.