We investigated the role of amphibian prey in the diet and distribution of the Aquatic Gartersnake (Thamnophis atratus) in the San Francisco Bay Area of California, USA. During surveys for amphibians and snakes at 185 ponds, we captured 139 T. atratus, of which 60 contained identifiable stomach contents. Native amphibians were found in 93% of the snakes containing food. Analysis of stomach contents indicated that Pacific Chorus Frogs (Pseudacris regilla) were the most important amphibian prey, followed by Western Toads (Anaxyrus [=Bufo] boreas), California Newts (Taricha torosa), and California Red-legged Frogs (Rana draytonii). The occurrence of T. atratus at a pond associated positively with the presence of all native amphibian species but negatively associated with the presence of introduced American Bullfrogs (Lithobates catesbeianus [=Rana catesbeiana]). The mean species richness of native amphibians at ponds where we detected T. atratus was also higher than that in ponds without Gartersnakes (2.45 vs. 1.74), and the odds of finding T. atratus at ponds with native amphibians was 12 times greater than at ponds without native amphibians. Our results underscore both the importance of native amphibians in the diet and distribution of T. atratus and the potential implications of ongoing amphibian declines for animals that prey on amphibians.