Pup mortality in the South African (Cape) fur seal Arctocephalus pusillus pusillus was measured throughout the pupping seasons over consecutive years (1987–1991) at two large mainland colonies in Namibia. Cumulative rates of mortality were obtained by counting live and dead pups every second day. Most young pups die shortly after birth in November/December each year, following which there is a decline in the rate of mortality. The average rate of mortality from birth to age 30 days at the Atlas Bay colony was 20 per cent. There was little inter-colony variation in the mortality rates of pups over the four years of the study. Differential rates of mortality between the sexes were found, significantly more female pups dying. Environmental factors are influential in the deaths of newborn pups at these mainland colonies, a combination of changes in wind velocity and direction leading to heat stress in pups being the most important.