The diet of the Texas map turtle (Graptemys versa) in the Colorado River drainage was studied using 1) stomach flushing and collection of feces in 1998–1999 and 2) dissection of museum specimens collected in 1949 prior to invasion of their habitats by exotic Asian clams (Corbicula spp.). Feces were found to be better indicators of dietary diversity based on log–log comparisons of sample volume with turtle size and the fact that Corbicula were absent in stomach flushes of small female specimens that yielded abundant shell fragments in their feces. Adult males fed most heavily on aquatic insect larvae (primarily trichopterans and ephemeropterans) and showed little change in diet over 5 decades. Small females that overlapped adult males in body size had wider heads and alveolar surfaces of the jaws than did males and fed predominantly on Corbicula, snails, insect larvae, and algae. Females that were larger than males in body size showed a change from a diverse diet in 1949 that included sphaeriid clams, trichopteran larvae, sponges, bryozoans, and algae, to nearly exclusive feeding on the invasive mollusk Corbicula 5 decades later. Based on the present report and previous studies, it appears that 1 result of the invasion of rivers in North America by Corbicula is greatly decreased diversity of diet in adult female Graptemys of moderate head width (mesocephaly).