Birth weights of South African babies were studied with reference to race, sex, incidence of prematurity, rank, age of mother and season of birth. 1. The mean birth weights were: European: 7.47 lb. Bantu: 6.77 lb. Coloured: 6.85 lb. Indian: 6.46 lb. 2. The incidence of prematurity was: European: 4.2% Bantu: 11.5% Coloured: 9.6%, Indian: 18.3% 3. Boys are heavier than girls at birth. 4. Birth weight increases with increasing rank up to 4+. 5. The effect of age of mother on birth weight is different for the racial groups. Increasing age produces a lower birth weight in Europeans and on the whole a higher birth weight for non-Europeans. 6. Seasonal variation in birth weight is only apparent in the higher birth ranks. It is more marked in girls than in boys, and in Indians than in the other races. Where the differences were significant, heavier babies were born in the summer months. It is suggested that sex and rank are very strong factors affecting birth weight in all races, and remain relatively unaffected by differences in socio-economic status and culture. The mean birth weights themselves, the incidence of prematurity, the seasonal variation and the effect of age of mother are all influenced by economics, nutrition and the cultural habits of the populations studied.