Mandrill Mandrillus sphinx hordes in the Lopé Reserve, Gabon, the approximate centre of the mandrill species range, were studied over 3 years from 1996 to 1999. Part of the study site included gallery forests within savanna areas, allowing observation of entire hordes, hitherto impossible in dense forest habitat. Horde size and composition (sex and age classes) were documented using exact records on video film whenever a horde or subgroup crossed an open space. Mean horde size was 620, and hordes of up to 845 individuals were documented, probably the largest stable group size found in any wild, unprovisioned primate population. Hordes were cohesive throughout the study period and did not seem to be aggregations of smaller units. Mandrill societies seem to be quite different from the baboon societies, to which they have been compared to date. Mature, breeding-age males were not resident members of hordes, but entered at the onset of seasonal cycles in the females (as deduced by the presence of sexual tumescence) and emigrated once female sexual cycles ceased. The number of breeding males present in the horde at any one time is best explained by the number of sexually attractive females. It is postulated that the extreme coloration of males and strong sexual dimorphism in mandrills may have evolved through an enhanced need for competitive signals in a situation where no long-term social bonds between breeding partners exist.