This study concerns five of the northernmost adder, Vipera berus, localities in the world that are situated in the basins of Tornio, Lainio and Vittangi rivers, between 300 and 450 m.a.sl. The study area lies approximately 150 km north of the Arctic Circle in northern Sweden. Three different kinds of slopes were used as hibernation sites, slopes on moraine ridges, slopes in canyons and on mountains. All were characterized by stony south facing slopes without topsoil and shading trees. Open areas of peat bogs and marshlands were always found within a kilometre's distance from the hibernation sites. Except pregnant females that remained around the hibernation sites, the adders were found feeding on voles in these areas from mid-June to mid-August showing a distinct shift in habitat use between seasons. The total length of the activity period was found to be 17-18 weeks.