The population of northern elephant seals (Mirounga angustirostris) on San Nicolas Island, California, was censused at monthly intervals from August 1968 through August 1971. An incomplete set of data from 1964 is also presented. The population had three yearly peaks as follows: an immature animal peak in November; a breeding season population peak in February; and a molting animal peak in April. The terrestrial population reached its lowest point of the year in August. All three peaks increased in magnitude during the period of this study. Total number of animals in the breeding peaks for the three years were: 1969, 393; 1970, 467; 1971, 702. Numbers of pups born were 193, 210, and 344, respectively. Numbers of animals in the molting peaks were 719, 776, and 1050, respectively. Estimates of growth curves for the breeding and molting peak populations are presented. Prediction of the number of pups born during each of the 3 years of this study yielded values approximately one-half of the actual number of pups born. Low female mortality rates or immigration of pregnant females, or both, may have accounted for a major portion of the pups born on San Nicolas. Pup mortality on land was low and was between 3 and 4 per cent in 1970 and 1971. Little information is available on mortality rates in older animals.