Comparative analyses of the biology of insular and mainland populations of mammals have demonstrated a number of behavioral differences. Individuals from island populations generally have reduced home-range sizes, increased territory overlaps, and reduced aggressiveness with neighbors in comparison to mainland counterparts. We tested the hypothesis that island and mainland populations of the swamp antechinus (Antechinus minimus maritimus), an insectivorous marsupial, will differ in their use of space. We predicted that the home ranges of individuals on an offshore island are smaller and their territory overlaps greater compared to those of antechinus at an adjacent mainland site in southeastern Australia. We used radiotelemetry to measure home-range areas and overlaps, as well as the temporal activity patterns of 40 individuals in the nonbreeding and breeding seasons at island and mainland sites. These dasyurid marsupials were social animals and nested together at both sites, and a high degree of spatial overlap was recorded in both populations. Island individuals occupied significantly smaller home ranges and were mainly nocturnal, whereas mainland individuals were diurnal and had large home ranges. The small home ranges of island individuals may have been in response to increased food resources, resulting from large allochthonous inputs from seabirds. The nocturnality of island animals was likely a predator-avoidance mechanism to evade diurnal raptors in the open tussock grassland.