Brushtail possums at a 21 ha site at Castlepoint in the Wairarapa, New Zealand, were studied with capture-mark-recapture from August 1989 to August 1994. The mean annual adult population density, based on counts of mature possums trapped each year, was 8.7 per ha and varied by only small amounts during the study period. The median survival age was 32 months (95% CI 28–39) for females and 27 months (95% CI 26–30) for males. Mean body weights were 2.47 kg (95% CI 2.46–2.49) for mature male possums and 2.34 kg (95% CI 2.32–2.36) for mature female possums. Body weights were lowest in autumn–winter, increased during spring and were highest during summer in each year. Breeding started in March each year and there was a secondary pulse in spring. No births were recorded in the summer. The median age for time to first successful mating for females was 14 months and almost all females bred each year. Rates for successful breeding in both autumn and spring ranged from 100% for the 90th percentile to 56% for the 10th percentile. The population contained more males than females throughout the study period but depopulation data showed a predominance of males in the age group of up to 4 years and similar proportions thereafter. The outstanding features of this population were its high density, high fecundity, breeding at an early age and a short life expectancy. It illustrates the widely varying location-specific performance of the species. Known causes of death included tuberculosis, iatrogenic haemopericardium and exposure–starvation. Juvenile possums often used dens that appeared to give poor protection during rain and cold conditions, and our observations suggest that a lack of dens which could provide protection from adverse weather was probably more important than abundance of food in regulating population density in the ecological setting at Castlepoint.