Demographic data were collected on one privately owned stud herd each of the Bonsmara, Brahman and Drakensberger beef breeds, farmed extensively in different natural environments in the Republic of South Africa, to study the influence of herd age-group structure on the rate of genetic gain and production. The number of calves weaned per 100 cows mated varied from 51 to 69 between different herds and cowage-group structures. All three herds required at least four cowage-groups to maintain population size and should attain maximum annual genetic gain with six cow and three bull age-groups. This age group structure secured almost maximum production in terms of calves weaned and weaning mass per cow mated. It is concluded that an age-group structure with six cow and two to four bull groups should allow near maximum genetic gain in most South African beef cattle studs with weaning rates between 60 and 70 calves per 100 cows mated. Herds with a weaning rate of 71 to 80% should contain five cow and two or three bull age-groups for maximum genetic gain.