Antechinus siuartii nest communally in natural cavities in trees. At Monga State Forest the commonest source of these cavities is dead trees (‘stags’) that foresters have ring-barked, leaving the trunks and major branches to rot. Young trees lack suitable cavities, which suggests that forestry directly influences the local abundance of this small mammal. Males always nest communally, and always with females. Maximum aggregation size was 18, during the rut. However, the aggregations are not stable, and during one winter a single nest attracted 28 females and 24 males. A subset of the communal nests was used during the mating season. Except for one case where a mother and daughter shared a nest during the lactation period, females nested alone to rear their young. Two females split their young between two trees. Females can also nest solitarily or in small female-only groups during a period of social turmoil in March and April, and less often in larger communal groups that persist through the rut. The failure of males to attempt to obtain copulations at these aggregations indicates that females directly influence where mating will take place. No nest was used for longer than 22 months, though abandoned nests are often later re-used. We suggest that the commonest causes of nest abandonment are avoidance of ectoparasites, change in the dominant habitat used by the animals, and the preference of females for a private nest to rear their young. In one important habitat type communal nests are never used to rear young. Nests used to rear young—‘natal nests’—were often used in consecutive years, though the frequency of re-use declined as the study proceeded. There was a shift in the habitat used by A. swartii. Old Eucalyptus fastigata forest declined in importance. By late 1989 animals reared their young in riverflats and Leptospermum swamps, habitat previously avoided. Daughters do not re-use their mother's nest. More sons are successfully reared from natal nests that will be re-used than from nests that will not, even though re-use depends on rediscovery of the nest by an unrelated individual.