Osmoderma eremita, an endangered beetle restricted to hollow trees, was surveyed in stands of different size, isolation and physical characteristics in an area in south-eastern Sweden. To infer habitat occupancy the presence/absence of fragments of adult body parts was used. Mark–release–recapture of adults with pitfall traps set in tree hollows was carried out to estimate local population size. Habitat occupancy/tree was higher in larger stands than in smaller stands, but there was no correlation between occupancy/stand and isolation of stands. This suggests dispersal is important in the population processes within stands, but not between stands. Osmoderma eremita was found in most of the larger stands and this probably mirrors the fact that the density of old-growth oaks has been much higher in the area than today. Habitat fragmentation over the last two centuries has disrupted the network of habitat patches, thus preventing dispersal into smaller, younger stands. A minimum number of suitable trees aggregated in stands seems to be necessary to provide long-term viability of local O. eremita populations. To preserve viable O. eremita populations, it is therefore necessary to increase size and connectivity of stands with hollow trees.