A population of baboons, Papio anubis, was studied for 3 months in an area of arid thornscrub in Ethiopia. The ecology of the habitat is described. Eight troops were surveyed, and one troop was studied in detail. The daily pattern of activity is outlined, and differential use of various parts of the habitat described. During movement and foraging the troop became scattered into small parties which might be spatially and temporally independent for several hours. Possible causes of such scattering are suggested, and party compositions are analyzed. The relationships between the various age-sex classes at a time when the troop is compact are investigated. These are shown to account for the types of party most commonly found when the troop is scattered. The relevance of this subdivision of the troop to current theories of the evolution and adaptive significance of one-male groups in P. hamadryas is discussed.