Population biology, behavioural thermoregulation and site preference of the New Zealand fur seal, Arctocephalus forsteri (Lesson, 1828) are described for the breeding colony on Taumaka, Open Bay Islands, Westland, New Zealand (43°52'S, 168°53'E), from data collected at irregular intervals between October 1974 and February 1977. Sixteen fur seals (6 females : 10 males) were shot and aged by counting growth rings in a canine tooth. The age determinations suggest that females produce their first pup at age 5 years, and that bulls attempt to hold territories by age 8 years, but rarely are successful until about age 10 years. During their delayed 'social maturation', bulls may gain experience necessary to maximize reproductive success once territorial. A bull's territorial success ultimately depends on his fighting ability, which depends largely on the presence or absence of lower canine teeth. Canines are broken off during fighting; bulls without lower canines rarely defeat an opponent. Females prefer sites on the rookery which provide pools of water, shade, and direct access to the sea. Both sexes use water and shade for cooling. The theoretical 'mean' pupping date on Taumaka is 10 December. The sex ratio at birth is about 1:1. Males are heavier than females at birth, and are heavier and larger than females at mean age 55, 140, 235 and 290 days. Pups are weaned at about 300 days. Pup growth rates differed dramatically between 1974/75 and 1975/76. For example, male pups were about 22 percent heavier at 235 days in 1975/76 than in 1974/75. Differential growth rates probably are a consequence of food availability to lactating females and differing environmental temperatures between seasons. Mortality to age 50 days is 20 percent. First year mortality may be 50, percent. Starvation may account for 70 percent of all deaths to age 50 days. Other causes of death include suffocation, drowning, trampling and predation. Human disturbance can kill pups indirectly by causing panic on the rookery. Tagging pups with monel metal cattle ear tags may increase mortality. About 1400 pups are born on Taumaka annually. Using this figure for pup production, and incorporating reproductive data from other fur seal species (where none exist for the New Zealand fur seal), the estimated population of pups, females of breeding age and bulls on Taumaka is about 3700. The estimated population size associated with Taumaka, either through birth or by breeding on the rookery, is about 6000-6500. Using these estimates, and incorporating population data from the Northern fur seal, the estimated population growth rate on Taumaka is about 2 percent per annum.