In a field study of three groups of wild long-tailed macaques, Macaca fascicularis, observations on social behaviour could be related to genetically determined paternity. In contrast to what has been found in many previous studies on captive primate groups, we found a relatively strong correlation between male dominance rank and reproductive success. In a large group the high success of the alpha male compared to other males could be explained only partly by his higher copulation score. His success also resulted partly from better timing of his copulations during maximum fertility of the females, in comparison with other males. We must conclude either that the alpha-male has more access to fertile females, or that females have a preference for the alpha-male during their maximum fertility. The females clearly displayed promiscuous behaviour. This behaviour implies a risk to a female that a male other than the alpha-male with proven qualities will become the father of her offspring. One expects that there must be a social advantage related to this female strategy. In the absence of indications of any direct social advantages to the female of this sexual behaviour pattern, the ultimate explanation for this female promiscuity is most likely is aggression reduction. This could be either through the devaluating of any single copulation, or through the confusion of paternity and a resultant reduction in the risk of infanticide.