Sexual conflict occurs when the evolutionary interests of females and males are divergent. Sex-differences in optimal copulation duration can be a source of conflict. Male red-sided garter snakes (Thamnophis sirtalis parietalis) produce a gelatinous copulatory plug during mating that occludes the opening of the female reproductive tract for approximately two days. The size of the plug is influenced by the copulation duration. We experimentally tested the contribution of male and female control over copulation duration. We ablated the largest basal spine on the male’s hemipene and found a reduction in copulation duration and an increase in the variation of plug mass. Further we anesthetized the female’s cloaca and found copulation duration increased in this treatment group as well. This suggests that males benefit from increased copulation duration while females actively try to reduce copulation duration. Therefore, sexual conflict is manifest in divergent copulation duration optima for males and females.