We present evidence of an opportunity to measure the strength of natural selection on body size through size-specific winter mortality of freshwater turtles. Data were collected from three overwintering sites along the Missouri River floodplain in central Missouri. Carcasses and live captures of four species indicated that mortality was nonrandomly distributed among species and may be explained by differences in overwintering behaviors. Analyses of mean body size within age classes for two species demonstrated standardized selection differentials ranging from 0.23–0.98 (standard deviation units) against small-sized (juvenile) individuals. Our study provides evidence of winterkill as a selective force on body size and of a possible adaptive mechanism for Bergmann's Rule.