A troop of olive baboons, Papio anubis, was studied for 11 months in an open-country habitat near Gilgil, Kenya. Detailed mapping of troop movements was undertaken and provided a precise picture of range usage, showing that a small part of the range was used intensively. Gradations of home range use are described quantitatively. Tendencies to spread out and coalesce in different parts of the range are investigated. Shifting ranging patterns are examined together with meteorological factors, seasonal food supplies and water availability.