Sex ratios for hatchlings and adult turtles collected in aquatic habitats were examined for red-eared sliders (Trachemys scripta elegans) collected at Long Lake in west-central Illinois. We found that sex ratios were initially balanced but became progressively more male biased with the passage of time. Large cohorts of newly recruited males seem to underlie the increasing male bias. Recruitment more than doubled between 2001 and 2004, and these turtles were strongly male biased. Climatic warming may have led to the current male bias. A period of warming at the site has allowed females to lay more eggs by lengthening the nesting season. Females are laying an extra clutch, which accounts for the increased recruitment. This clutch is laid when soil temperatures are relatively low, explaining the male bias in newly recruited turtles. The impact of the increased number of male turtles on the population is uncertain. However, female condition declined about 7% between 1994 and 2006, suggesting that an effect may be occurring.