Many reptiles exhibit temperature-dependent sex determination (TSD) in which the incubation temperature of the egg determines the sex of the embryo. The purpose of the current study was to examine intraspecific variation in temperature sensitivity during TSD. Two aspects of temperature sensitivity were evaluated in the Red-Eared Slider, Trachemys scripta: (1) clutch-specific variation in sex ratios produced at pivotal temperature and (2) variation in clutch sex ratios produced over the nesting season (e.g., early nesting season, midnesting season, late nesting season). Relatively large numbers of clutch sex ratios were examined from two or three time periods (e.g., early nesting season, midnesting season, late nesting season), during each of three nesting seasons. The results indicate that pivotal temperatures vary significantly among clutches, with clutch sex ratios ranging from all male to all female when incubated near the putative pivotal temperature. The results on seasonal variation in clutch sex ratios were more ambiguous. Data from two of the nesting seasons do not reveal significant variation between time periods. During the third nesting season, sex ratios from the first two time periods were not significantly different from one another, but the sex ratios from the very end of the nesting season were significantly different from those of the two earlier time periods. In summary, the results reveal a clutch-specific variation in pivotal temperatures, but the results from the seasonal study of sex ratios did not consistently reveal a shift in sex ratios over each nesting season. Copyright 2006 Society for the Study of Amphibians and Reptiles.