Alterations of flow regimen, pollution, and introductions of exotic species have significantly altered the composition of invertebrates in many river systems throughout the world. How these alterations affect the diet of higher level predators is not well understood. We studied the diet of the Northern Map Turtle (Graptemys geographica) in the dam-regulated Susquehanna River in north-central Maryland. Northern Map Turtles are a relatively large, top-order predator that is legally endangered in the state and is impacted by commercial collecting elsewhere. Gastropods, trichopteran larvae, and invasive Corbicula sp. predominated across diet samples. Marked sexual and size-related differences occurred. Adult male G. geographica fed primarily on a group of small gastropod species (Planorbidae, Hydrobiidae, Physidae), trichopterans, and Corbicula, while adult females fed primarily on pleurocerid snails. There was virtually no overlap in the diets of the two sexes of G. geographica. This is of special conservation concern because two different groups of prey are needed in order to support this population of Northern Map Turtles, and many North American pleurocerid gastropod species are highly endangered and also threatened by hydroelectric activity. Our results are consistent with reports of other Map Turtle populations before the invasion of Zebra Mussels (Dreissena sp.), which often result in a sharp change in diet. Zebra Mussels have already been documented immediately upstream of our study site, so impacts from this invasive species may become apparent in the near future.