Technical Abstract: Mating pairs of Anthocoris whitei Reuter (Hemiptera: Heteroptera: Anthocoridae) irregularly exhibit prolonged copulations exceeding 5 hrs in duration. Atypically long copulations may act as a form of post-insemination mate guarding to prevent insemination of a guarded female by other males. In many insect taxa, male-biased sex ratios or high densities of male conspecifics lead to prolonged copulation. We tested whether exposure to conspecific males preceding access to females led to prolonged copulations by A. whitei. Copulations in which the male had been exposed to other males were significantly longer (by 60+ min) than copulations in which the male had not been exposed to other males. Durations exceeded 5 hrs in several pairings following exposure to other males. Almost 75% of copulations in which males had not been exposed to other males were less than 100 min in duration; in contrast, only 22-29% of copulations involving males that had been exposed to other males were less than 100 min in duration. These results support the idea that male-biased sex ratios preceding access to females prompt post-insemination mate guarding by males of this species.