Body temperature (Tb) of socially hibernating alpine marmots, a pair and two family groups, was monitored continuously from October to March with implanted temperature-sensitive radiotransmitters. At the same time, the animals' behaviour was observed. The recurrent entrances into and arousals from hibernation were highly synchronised within groups. Group members always lay huddled together when euthermic and also when torpid with a few exceptions at higher ambient temperatures (Ta). Body contact with euthermic nestmates warmed torpid marmots passively. The Tb of animals reentering hibernation did not fall to values close to Ta as long as euthermic group members were present. Although animals presumably save energy through social thermoregulation, especially when euthermic, these benefits are not necessarily mutual among group members. Differences in thermoregulatory behaviour of individuals described in this study could be responsible for differential weight losses during winter as found in the natural habitat (Arnold 1986).