Very large and stable, socially coherent primate groups, not including fission-fusion societies, are usually rare in nature, owing to constraints imposed by various ecological and social factors. Moreover, unlike species in open habitats, those in forests tend to have smaller groups, and this becomes further accentuated in small and fragmented forest patches. We report here an unusually large troop of stump-tailed macaques Macaca arctoides from the Hollongapar Gibbon Sanctuary, a small and isolated lowland tropical rainforest patch in the Upper Brahmaputra Valley of northeastern India - this is possibly the largest wild group of the species recorded anywhere across its distribution range. We hypothesise the potential factors driving the formation of such a large social group of this vulnerable cercopithecine primate and discuss the conservation implications of this phenomenon.