We compared incidence and average size of melanistic, adult male Trachemys scripta among three populations in Oklahoma. Proportion and size of melanistic males differed among two uncontaminated sites and one site contaminated with heavy metals. Average size of melanistic animals collected from one locality were larger than melanistic animals collected at the same locality 50 years ago; however, minimum size of melanistic males at all sites was consistent with published values for populations of T. scripta throughout its range. The proportion of melanistic males among the three sites differed significantly and the smallest proportion of melanistic males was in a population of T. scripta from Tar Creek Superfund Site, Ottawa Co., Oklahoma, an area contaminated with heavy metals.