Both color (i.e., reflectance) and body size should influence body temperature in heliothermic ectotherms. We compared heating rates and equilibrium temperatures between the normal striped morph and the solid black melanistic morph of the garter snake, Thamnophis sirtalis, from polymorphic populations in the Lake Erie region. Snakes ranging in mass from 2.6–216.2 g were divided into three size classes (small, medium, and large). Using temperature-sensitive PIT tags, we tested pairs of snakes under artificial light in an environmental chamber, then used curvilinear regressions to estimate heating rate and equilibrium temperature. In small and medium-sized snakes, we found no difference in heating rate nor equilibrium temperature between morphs, suggesting that melanism does not provide a thermal advantage in young snakes. In large snakes, morph affected equilibrium temperature, but not heating rate. Melanistics averaged 1.3 C warmer than striped snakes. This difference may represent a thermal advantage for adults, of which gravid females are the largest and may have the most to gain from a thermal advantage.