The diet of the Common Map Turtle (Graptemys geographica) was studied in Lake Erie habitats associated with a Pennsylvania sandspit peninsula. Three taxa predominated in fecal samples: trichopteran larvae (principally leptocerids), gastropods, and Zebra and Quagga Mussels (Dreissena spp.). Pronounced sexual and size-related differences occurred for these taxa, as adult males fed primarily on trichopterans and snails, while the larger females fed almost exclusively on Dreissena spp., with prevalence of Dreissena spp. increasing with body size. Dietary data from habitats lacking invasive mollusks suggest that the overall reliance on mollusks in the diet may have increased since the invasion of Lake Erie by Dreissena spp. Results are similar to earlier reports concerning the effect of invasive Asian Clams (Corbicula spp.) on decreasing dietary diversity in females of many populations of the four species of Graptemys that have mollusk-adapted mesocephalic females.