The synchrony of behaviour among individuals in a group of Yakushima macaques was evaluated in relation to group size and food resources. The degree of synchrony was greater when the group was small (5–8 individuals) than when it was large (15–19 individuals). Diet did not affect the degree of synchrony. However, the duration of the ‘active phase’, in which most members of the group synchronized feeding and moving, was shorter when they fed mostly on fruit and seeds (fruit- and seed-eating season) than when they fed mostly on leaves and fallen seeds (leaf- and fallen-seed-eating season). When the group was large, the monkeys ranged over a greater area and foraged in fewer trees during the fruit- and seed-eating season than during the leaf- and fallen-seed-eating season. However, this tendency was not so clear when the group was small. These results suggest that the extent to which the distribution of food resources determines patterns of foraging increases with group size and that monkeys in a larger group reduce levels of intragroup food competition in order to obtain sufficie