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Bullfrogs (Rana catesbeiana) were introduced into California about 100 years ago and are common in permanent and semipermanent aquatic habitats, except those at high elevations and in some desert areas. In the Great Central Valley of California, urban and agricultural development has led to increased hydroperiods in areas that historically supported only ephemeral sources of water. This has increased breeding opportunities for bullfrogs and expanded their dispersal into grassland and vernal pool ecosystems. Because of their voracious feeding habits, the diet of bullfrogs has been well studied, and bullfrogs have been implicated in local declines of some native species. Little is known, however, about prey selection by bullfrogs in ephemeral ecosystems such as vernal pools. We studied aquatic prey availability and the diet of juvenile bullfrogs at a constructed vernal pool wetland mitigation site in Sacramento County, California from February to June, 1998. Early in the spring, bullfrogs were widely distributed on the study site, but as pools with shorter hydroperiods dried, they dispersed away from the site or moved to larger, deeper pools. In pools occupied by bullfrogs, aquatic macroinvertebrate samples were dominated by crustaceans and coleopterans early in the spring, and by hemipterans later in the spring and early summer. The majority of the juvenile bullfrogs' diets consisted of aquatic macroinvertebrates, but terrestrial coleopterans, hymenopterans, dipterans, odonates, and arachnids were also preyed upon. Aquatic prey selection was non-random, with crustaceans and beetles over-represented and hemipterans under-represented in the diet compared to their abundance in the pools.