Using an index of relative importance we analyzed the stomach contents of over 300 water snakes (Nerodia spp.). Ontogenetic changes in prey consumption are most striking in Nerodia erythrogaster (number with food in gut = 44) and N. fasciata (N = 72). Prey of these two species changes from fish to frogs as the snakes exceed a snout-vent length of 50 cm. Nerodia rhombifera (N = 159) and N. cyclopion (N = 65) primarily eat fish throughout their life. However, with maturity and increased body size both species change portions of their diets. Nerodia rhombifera preys upon larger fish which occupy deeper, open-water habitats, when the snakes exceed 80 cm. Nerodia cy- clopion eats a larger proportion of centrarchid fish as its body size increases. Small prey are found in the stomachs of most size-classes of all four snake species. Regression analysis indicates that all four species eat larger prey as they mature. However, the largest individuals are females, and in two of the four species the large females eat a different array of prey than smaller nonspecific males. The size sexual dimorphism does not reduce the overlap in the diets of the two species that eat anurans as adults.