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Male displacement of copulatory (sperm) plugs from female vaginas provides further evidence for sperm competition in ring-tailed lemurs (Lemur catta), a gregarious prosimian species with a multimale, multifemale mating system. During two mating seasons, I studied two groups of free-ranging ring-tailed lemurs on St. Catherines Island, GA, USA. I observed 22 mating pairs in which males achieved penile intromission. Copulatory plug displacement by males occurred in 9 cases. Plugs were displaced during copulation by male penes upon withdrawl following deep vaginal thrusting. In every case of copulatory plug displacement, the male displacing a plug mated to ejaculation with the estrous female. In a mating system in which females typically mate with more than one male during estrous, often in succession, copulatory plug displacement may function to disrupt or preclude other males' successful insemination of estrous females. The effects of sperm plug displacement on paternity in Lemur catta are unknown, as no study had heretofore documented copulatory plug displacement in this species. The first-male mating advantage suggested for Lemur catta should be re-evaluated where mating order is known, and copulatory plug displacement during mating, or lack thereof, is identified. Because there is a tendency for first-mating males to mate-guard for longer periods of time in Lemur catta, the latency period between the first mate's ejaculation and that of subsequent mates may be an important determinant of male fertilization success.