Mating preferences, including the proximate mechanisms of preferences, have not been well studied among parasitoid wasps. In the parasitoid wasp Spalangia endius Walker (Hymenoptera: Pteromalidae), males chase after both mated females and virgin females equally. Not until contact or near contact are males more likely to retreat from mated females and hence less likely to mount them. This study used a behavioral perspective to test several hypotheses about the proximal cause of such retreats. Retreats do not seem to be a response to aggressive physical behavior by mated females. Male retreats also were not just a response to female motion; females were not consistently moving or consistently still before male retreats. Retreats were not simply a response to antiaphrodisiac pheromone on the surface of mated females; males did not avoid mounting dead females. If a pheromone is involved, females seem to be actively releasing it, in contrast to the antiaphrodisiac surface pheromones known in other hymenopterans. The mated female’s head and thorax, but not her abdomen, were essential to unattractiveness at the mounting stage of mating.