The flying squirrel Pteromys volans is an arboreal rodent and inhabitant of Palearctic boreal forests. In Finland, the flying squirrel has been classified as a declining species which needs to be monitored. I studied home ranges, habitat use and nocturnal activity of eight adult flying squirrels by radio tracking in fragmented coniferous forests in Finland during June - December, 1996. Average home-range size of the flying squirrel measured by the 100% MCP was 6.5 ha. In summer, the average size of the 95% cluster area was 2.3 ha and the 80% core area 0.5 ha. The core areas represented only 7.8% of the 100% MCP area and were composed of 2–6 separate patches in the home ranges of individual squirrels. Radio-tagged squirrels used several nests, both old woodpecker cavities and dreys for nesting and diurnal roosting. The combined density of all deciduous tree species was significantly greater in the 80% core areas than within the 100% MPC in the summer data set. In the polychotomous logistic regression model the great canopy cover, high densities of alders Alnus incana and A. glutinosa and aspen Populus tremula significantly explained the ranked utilisation classes (utilisation rank from highly used areas to least used areas: 80% core - 95% cluster - 100% MCP). The three most abundant deciduous trees species (birches Betula pendula and B. pubescens, aspen, alder) constituted 87% of trees used by squirrels in summer. Flying squirrels were found in aspens more often than expected according to their availability. The results show a clear preference for deciduous trees and a preference for the parts of home ranges with higher densities of alders and aspen. The flying squirrel seems to be capable of using several cover types, including young forest stands, as foraging and moving areas and are able to move across semi-open clear-cut areas.