A dormouse found in hibernation in its winter nest on January 26 was studied continously from February 5 until May 11 by recording ambient temperature, temperatures inside the nest ball and 5 cm from it, and by recording any possible motor activity. The first emergence from hibernaculum occurred on April 3 after which the animal was active each day with the exception of April 11, 13 and 14. Activity mainly occurred during evening and night hours and lasted on average 4 hrs (2-8 hrs) per day. Outside periods of activity the winter nest was consistently used as a place of shelter and for sleep.The ambient temperature ranged from-0.5° to 21.0°C being chiefly 1°C less the nestbox temperature. The difference between the nest-box and nest temperature was also about 1°C when the animal was inactive, thus clearly indicating torpidity. Steep increases in nest temperature, amounting to 14-18°C and raising nest temperature up to 30°C, were recorded on four occasions. This is interpreted as shallow torpor, since no activity occurred on these days.The spontaneous warming up from deep hypothermia to shallow torpor lasted on average 40 min (30-70 min), while the duration of passive cooling when returning to the hypothermic condition amounted to 5 hrs. In the weeks following continuous hibernation the dormouse alternated between activity, shallow torpor, and relatively deep torpor each day. The species should be considered as a true aestivator.