Denning groups (colonies) of Leadbeater's possum Gymnobelideus leadbeateri inhabiting nest boxes in lowland swamp forest were characterized by the presence of only a single breeding pair. The percentage of adult females and males in the population that held the position of ‘resident breeding individual’ within their colony was 66±12 and 62±13%, respectively. The remaining animals were mainly younger adults yet to acquire breeding positions within established colonies. Male and female Leadbeater's possums that attain breeding status within a colony usually reproduced for more than 3 years. Breeding vacancies that arose following the death of an individual were quickly filled by immigrants from neighbouring colonies. Typically, both male and female possums did not commence breeding until they were more than 2 years of age and had dispersed from their natal colony to a neighbouring group. Births were recorded in every month, with no discernible peaks in reproductive activity. This is the first reported case of aseasonal reproduction by a small petaurid species in temperate Australia. The mean litter size was 1.55, and females typically produced two litters per year. The reproductive ecology of Leadbeater's possum differs from that of other small (<260 g), communally denning petaurids in temperate Australia in the following respects: (1) first breeding occurs when individuals are older, (2) breeding occurs throughout the year, (3) the natality rate per female is higher and (4) at any time, reproduction appears to be restricted to a single pair of animals within each colony.