We studied the diet of the exotic American bullfrog (Lithobates catesbeianus) in Arroyo San Carlos, located in northwestern Baja California, Mexico, during spring and summer 2009. Analysis of 64 stomach contents revealed 15 prey types, of which the exotic red crayfish (Procambarus clarkii) and terrestrial isopods (Armadillidiidae) were the most significant items (according to an index of their relative importance), making up 64% and 31% of the bullfrog's diet, respectively. Diet differed between the sexes, with red crayfish eaten more frequently by males (84%) than females (69%). Bullfrog diet changed with body size, with smaller individuals (≤86 mm) containing woodlice (61%) as the dominant prey type and larger individuals (≥129 mm) containing red crayfish (97%) as the dominant prey type. The average prey size consumed was significantly correlated with body length.