The Mexican Pacific lowlands garter snake, Thamnophis validus, is found in aquatic habitats on the Pacific coast of México from the states of Sonora to Guerrero and in the Cape Region of the Baja California peninsula. We characterized the diet of Thamnophis validus through examination of stomach contents of museum specimens and from literature records. The diet of T. validus consists almost entirely of fishes and larval and adult anurans. There is strong geographic variation in the diet: snakes from uplands of Baja California contained only anurans, whereas the majority of snakes from mainland México that contained prey had eaten fishes. Mainland snakes show a pronounced ontogenetic shift from feeding primarily on anurans to feeding primarily on fishes. When mainland snakes are divided into three size classes, the intermediate class shows the most generalized diet (i.e., the most even distribution between fishes and anurans). We suggest that this nonmonotonic relationship between predator size and the level of specialization may be relatively common in snakes. Prey mass and the variance in prey mass both increase with increasing predator size in T. validus, as is the case for many carnivorous animals. Thamnophis validus apparently differs from other aquatic garter snakes (T. atratus, T. couchii, T. hammondii, T. melanogaster, and T. rufipunctatus) in feeding heavily on adult anurans and in the common use of aquatic open-mouth searching behavior.