We studied the behavioral thermoregulation of Japanese macaques in two troops that live in the coniferous (1,000-1,200 m in elevation) and coastal forests (0-200 m in elevation) of Yakushima. Frequency of sunbathing, huddling, and microhabitat selection during inactivity was compared. The difference in mean annual air temperature between the forests was more than 7 degrees C. In both forests, when the weather was clear, macaques spent more time being inactive in the sunshine in winter than in autumn. In winter, they huddled more often when it was clear than when cloudy. Microhabitat selection to stay in the sunshine during winter differed between the two forests. In winter, macaques spent more time inactive in open habitats in the coniferous forest and in the trees in the coastal forest than in autumn, respectively. This difference is related to the lower crown height in the coastal forest and the large open habitats (logged area) available only in the coniferous forest. In winter, skin temperature measured by temperature-sensitive transmitters was 1.32-1.71 degrees C higher when sunbathing, and 0.83-4.75 degrees C higher when huddling than staying in the shade without huddling. In winter, the proportion with which they stayed in the sunshine or huddled in winter did not differ between the two forests, in spite of the difference in air temperature. This suggests that Japanese macaques respond to seasonal changes in air temperature, not the absolute temperature, and that they acclimatize themselves to thermal conditions that require behavioral thermoregulation only during the season when thermoregulation is most costly.