Male slider turtles change coloration ontogenetically, becoming melanistic as they age and approach the body size of sexually mature females. Several investigators have suggested that melanism is favored by sexual selection, although there is little evidence to support this hypothesis. We conducted experiments in enclosures to test whether melanism in male Red-Eared Sliders is a visual signal favored by sexual selection. In female preference trials, females showed no preference for either nonmelanistic or melanistic males. Melanistic and nonmelanistic males associated together instead of being aggressive toward each other in trials testing male–male competition. Neither result supports the hypothesis that male melanism is a product of sexual selection. Our observations of nonaggressive interactions between melanistic and nonmelanistic males suggest that melanism in male slider turtles may function as sex recognition to avoid mistaken courtship of large males.