The diet of the Concho water snake Nerodia harteri paucimaculata was investigated from 1987 to 1990 by palpation of stomach contents. Prey remains representing 304 prey items were recovered from 192 individual snakes. Concho water snakes were almost completely piscivorous, feeding on 19 species of fish from nine families with minnows (Cyprinidae) dominating numerically. Cricket frogs (Acris crepitans) represented the only non-piscine prey. Diet diversity increased ontogenetically according to snake body size. Observations of foraging snakes suggest an ontogenetic change in foraging sites concurrent with a change in diet composition. Prey size was positively correlated with snake body size although some snakes occasionally ingested numerous small prey, possibly due to opportunistic feeding on small prey aggregations. Snakes occasionally attempted to handle prey too large to be ingested. Feeding occurred from mid-March to early November. Gravid females fed throughout the spring into mid-July and resumed feeding after parturition. Neonate and juvenile riverine snakes ingested prey in proportion to apparent availability while adults consumed a disproportionate amount of larger prey species. Lacustrine snakes primarily consumed prey associated with benthic or shallow water habitats. However, few individuals of open water and top water species were ingested, suggesting that prey habitat preference strongly influences catchability in lake systems.