Few systematic field observations of the behaviour and ecology of the Chacma baboon, Papio ursinus, have been reported since Zuckerman (1932). The present study has set out to obtain reliable evidence on all aspects of the behaviour of natural-living groups of these animals in two regions of their extensive distribution in Southern Africa—the arid thornveld of South-west Africa and the temperate area of the Cape Peninsula. Field work was carried out in two parts: (1) regular observations of groups of baboons in the Cape of Good Hope Nature Reserve for a period of one year; (2) intensive observation (200 hours) of a single group in the Reserve, as well as extensive surveying of groups in the mountainous regions of South-west Africa. The present paper gives numerical data on thirty-one groups, analysis of group constitution, day-ranging distances and sizes of occupied areas, times of leaving and returning to sleeping-cliffs, and other data on routine activities. Particular attention has been paid to schedules of food and the description of feeding-techniques, and field observation has been supplemented by some simple experimental tests on the eating of scorpions and other creatures. Close range observations were made of baboons eating shell-fish, and some evidence is presented on the acquisition of feeding habits through observational-learning (imitation).