Factors affecting body temperature (Tb) fluctuations in winter were examined in Korean field mice Apodemus peninsulae. Animals were maintained under the following three conditions to mimic autumn followed by winter, plus two modes of behaviour, solitary and grouped, in winter: 1) short day (8 Light:16 Dark) at 25 ± 2°C and solitary, for 12 weeks towards the end of which their Tb was recorded; 2) constant darkness (DD) at 4 ± 2°C and solitary, for a week; and 3) DD at 4 ± 2°C and in groups (two or three animals in a cage), for about 10 days. Using implanted data loggers Tb was monitored in 13 mice that were exposed to these three conditions in sequence. When the animals' body temperature was below 30°C, it was assumed that they were torpid according to the bimodal pattern of minimum daily Tb. The maximum period of daily torpor was 390 min/day, and torpor was exhibited by seven of the thirteen animals exposed to conditions-2) and -3). Since torpor was found in one mouse even under condition-1), without any environmental stressors, it is suggested that this species is prone to spontaneous daily torpor. Cohabitation tended to synchronize Tb fluctuations irrespective of the state of the animals, torpid or not. Thus, this species may aggregate and synchronize torpor and/or Tb fluctuations to reduce heat loss during winter.