A wild troop of Papio papio was observed over a 3-month period in Senegal, West Africa. The overall organization of the troop is described, and it is suggested that some kind of substructuring occurs within the troop. An analysis of the compositions of night- and day-resting parties confirms this, but indicates that the way in which the troop splits up varies according to the time of day. Vigilance behaviour and troop organization during travel are discussed and compared with findings by Bert et al. (1967a, b] on troops of P. papio in other parts of Senegal. The observations are discussed with reference to similar findings from other species of monkeys in Africa.