Influence of habitat type on the group size of sika deer was analysed on the basis of 2718 individuals observed in 708 groups, in the area where deer are under pressure neither from predators nor hunters. From spring to autumn, the percentage of individuals observed in the largest groups in open woodland was higher than both in clearings and closed woodland. In early winter, the percentage of individuals aggregated in the largest groups was highest in the clearings, while in late winter, the percentage of individuals observed in the largest groups was highest in the closed woodland. Besides, the percentage of large groups observed in the morning and evening (when most of deer are active) was much higher than during daytime (when level of deer activity is lower). Changes in food availability are suggested to be a factor responsible for variation in sika deer group size in different habitats.