The small population and limited range of the Samoan flying fox Pteropus samoensis has generated concern regarding the survival of this large, diurnally active bat. During 1995–96, surveys were conducted monthly in six study valleys on Tutuila Island, American Samoa, to assess population size. The amount of diurnal and nocturnal activity was investigated to gauge the accuracy of diurnal surveys, and territorial behaviours were observed to determine how they influenced local dispersion. Individuals showed long-term fidelity to a series of roosts and small core areas that were used both nocturnally and diurnally. Territorial defence was observed only of temporary feeding territories in fruiting or flowering trees. Bats defended food resources by aerial patrols and extended aerial chases in which intruders were frequently bitten. Foraging movements changed seasonally, with up to 80% of individuals observed bypassing study valleys. The mean density of bats observed within the study valleys was 6.1 bats/km2 (range = 0.9–18.5 bats/km2). Pteropus samoensis were active both nocturnally and diurnally with greatest activity in late afternoon and evening, 16:00–22:00. Because bats were most active at night, it is probable that daytime surveys of flying bats undercount the number of individuals present. Greatest densities were found in valleys that were contiguous with large tracts of forest inaccessible to people. Most observations of roosting bats were of solitary males on dead branches that jutted above the forest canopy, while females and dependant young roosted below the canopy, hidden within vegetation. Adult male–female pairs were rarely seen together other than during the mating period in August–January. The population has increased following a ban on hunting, but reliance on mature forest makes long-term species survival dependant on protection of the limited mature forest remaining and continued hunting restrictions.