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Behavioural and endocrine parameters were studied in a large colony of stumptail macaques. This paper deals with harassment behaviour during copulation, and its possible underlying motivation. The study corroborates some findings of other studies such as that harassment is triggered by ejaculation and is directed at the mating male. Both males and females harassed most frequently around puberty, but only females continued to harass when adult, although at a lower rate, especially when pregnant. Harassment was seldom retaliated, even when severe aggression to dominant mating males was involved. Social factors that correlated with harassment included family ties, affiliative ties and reciprocity between harasser and female copulant, dominance status and aggression between harasser and male copulant. Motivational factors appeared not to be exclusively associated with the harassers' attitude towards the female copulant, as other studies suggested, but also involved the harassers' attitude towards the male copulant since the behaviour was invariably directed at him. In addition to possessiveness, it is suggested that revanchism may be a motive. Because of aggression received earlier, monkeys take ‘revenge’ when they have an opportunity, i.e. when the opponent is mating and at the moment of orgasm. It is suggested that, as an evolutionary mechanism, harassment reinforces power structures in the group. Harassers may unwittingly serve as sentinels for top-ranking males who thus become better able to monopolize fathership. As an additional effect, high-ranking matrilines can grow at the cost of low-ranking ones because of the preference of top-ranking males for high-ranking females.