Many amphipods (Crustacea: Amphipoda) exhibit sexual dimorphism in the size of a paired chelate appendage, the gnathopods. As with sexually dimorphic weapons and ornaments in other taxa, little is known of the evolutionary processes underlying species variation in the dimorphism. In this study I examined males of two ecotypes of freshwater amphipods in the genus Hyalella to determine (1) the degree and form of reproduction-mediated selection acting on gnathopod size, (2) the importance of fluctuating asymmetry in gnathopods for mating success and (3) differences between ecotypes in gnathopod size. I examined sexual selection arising from differential success in forming mating pairs and natural selection due to success in mating with larger, more fecund females. The two ecotypes differed in the pattern of selection acting on gnathopod size. The small-bodied ecotype did not experience significant selection on gnathopod size in either selection episode. In contrast, the large-bodied ecotype experienced significant sexual selection for increased gnathopod size (relative to overall body size) and experienced selection for smaller gnathopod size during the natural selection episode. Fluctuating asymmetry in gnathopods occurred at relatively low levels and was not related to pairing success in either ecotype. The two ecotypes differed significantly in gnathopod size relative to body size, with the small-bodied ecotype having gnathopods approximately 18% larger than those of the large-bodied ecotype. These results, together with previous results, suggest that gnathopod size is important in the mating success of males of the large-bodied ecotype, but not the small-bodied ecotype, and that ecotype differences in the level of the sexual dimorphism may have evolved for reasons unrelated to mating success.