The age structure of samples of possums from 11 areas in New Zealand varied seasonally, mainly in response to the major birth pulse in May. Between May and September, 1–2 year-olds predominated in trapped or poisoned samples. Between October and early January, 6–12 months-old animals usually predominated. Animals under 2 years old provided the bulk of most samples. Of 2492 females aged by counting annual cementum layers on teeth, 4 reached the age of 14 years. On leaving the pouch, possums in the Orongorongo Valley, Wellington, had a life expectancy of about 6 years, mortality being lightest in the animals 2–4 years old. The average annual mortality rate of adult and subadult animals in the Orongorongo Valley was about 15%. Males outnumbered females in the 0–2 year age classes in 9 of 12 samples analysed and, in pooled samples, 0–1 year-old males outnumbered females by 135–100. This disparity is attributed to a greater proportion of males in the pouch and a trapping and poisoning bias in favour of males. Females increasingly outnumbered males in the over-7-year age classes. In the Orongorongo Valley 44% of natural deaths of adults and subadults occurred between June and August and the number of animals found dead or dying ranged from 4 to 62 in their best and worst years.