Based on the stomach content analysis of 113 individuals, the diet of the invasive amphibian Lithobates catesbeianus (American Bullfrog) was examined in four sites located within the municipality of Viçosa (20°45′S and 42°51′W), state of Minas Gerais, Brazil, from August 2005 to March 2007. The effects of frog size and sexual maturity on stomach contents were determined. Prey items were grouped according to their primary habitat, being classified as aquatic, terrestrial and amphibious. In general, the most frequent prey categories were post-metamorphic Anura, Diplopoda, Hemiptera, Hymenoptera Formicidae and Araneae. The diet of adults of both sexes was similar, but differed from the diet of young frogs. Terrestrial prey were most abundant both in number and occurrence. For adult Bullfrogs, amphibious prey were most significant in volume. There was a significant correlation between prey and predator sizes, as well as a greater consumption of native anurans by larger Bullfrogs. The results confirmed that Bullfrogs have a generalist feeding habit that can have important negative effects on native amphibian communities in environments occupied by this invasive species.