Adult females of the toad Bufo marinus significantly exceed adult males in snout–vent length (SVL), but at any SVL, adult males greatly exceed adult females in the robustness of the forelimbs. Secondary sexual shape dimorphism results from greater size and mass of the skeletal elements of males, and especially from their hypertrophied forelimb musculature. The dimorphism is particularly pronounced for those elements of the musculo-skeletal apparatus known or presumed to function in amplexus, and this intersexual difference is hypothesized to have evolved under sexual selection. Reproductively successful males (those found in amplexus) significantly exceed unsuccessful males (calling, nonamplectant males) in SVL, an example of the large-male mating advantage commonly seen in anuran amphibians. However, independent of body size, amplectant males significantly exceed nonamplectant males in forelimb muscle mass, suggesting that it is not large body size per se that confers a mating advantage but rather the robustness of the forelimbs allows males to retain a firm grip on the female in amplexus and, thus, to resist take overs by competing males in this explosively breeding species. The intrasexual component of Darwian sexual selection, expressed as male-male competition, is implicated in the evolution of this sexual dimorphism, but a role for female choice cannot be excluded. The results are consistent with the hypothesis that intersexual differences in forelimb muscle mass have evolved under sexual selection, and they suggest that, for males, selection favoring large forelimb muscle mass is operating still.