A group of 12 wild-born adult stumptailed macaques and their 10 offspring, housed in an outdoor enclosure at Stirling University Primate Unit in Scotland, was intensively studied over two years and 271 copulations to account for vigorous harassment of mating pairs observed in this species. Three hypotheses were tested: that harassment (a) functions to protect the mating female from aggression by the mating male; (b) functions to reduce the reproductive potentials of mating monkeys; and (c) is motivated by possessiveness of a mating individual. The Protective Hypothesis was rejected. The Reproductive Potential Hypothesis received support only in that any male dominant to a mating male could consistently interrupt copulations. All other harassers not only failed to disrupt copulations, but harassed mating kin more than comparable non-kin. The Possessive Hypothesis received support: (a) females directed more harassment at matings of females, but not males, with whom they were most highly affiliative; males showed no such tendency; (b) the more desired contact partners, dominant adults, received more harassment than less "attractive" individuals; and (c) post-mating interactions indicated that normally strong bonds between mating monkeys and their usual companions were disrupted by the matings. In matrilineal Old World monkey groups, since the most enduring bonds arise between females, the function of harassment by females appears to be reinforcing access to allies, kin, and dominant individuals.