The age-sex composition of a chacma baboon (Papio ursinus)population changed following a 5-month interval of extreme food and water shortages. Mortality was significantly greater among adult females, juveniles, and infants than among adult males. The probable basis of 19 of 22 deaths during the interval of food and water shortage was starvation caused by drought conditions which localized water sources, reducing access to food resources. This resulted in a long-term (> 6-year) shift in adult sex ratios within this three-troop population, from 1.04 to 1.42-1.58 adult and subadult males per adult female. Patterns of intertroop interaction were also influenced by food scarcity, which determined which troop was most seriously affected. Keywords: chacma baboon, demography, population regulation, sex ratio, intergroup conflict.