Field sampling and laboratory experiments were used to examine the mating system of an anomuran crab, Pachycheles rudis, which typically occurs in heterosexual pairs. Paired crabs occurred over a large size range and were strongly size correlated, suggesting the possibility of long-term pairings. Moreover, both sexes exhibited aggression towards conspecific strangers of the opposite sex. Crabs were separated from their mates for periods of 24, 48, 72, 96, or 120 hours, and their behavior monitored following reintroduction. Resident males showed an elevated level of aggression towards their returning mates following periods of separation greater than 48 hours, and the number of pushes and other acts of aggression were comparable to that shown against strange females. The formation of new pairings involves agonistic interactions between sexes, and females appear to be dominant in stable pairs. Pachycheles rudis appears to fit models that were previously constructed for monogamous Crustacea remarkably well.