The socio-ecology of representative groups of tantalus monkeys (Cercopithecus aethiops tantalus) was observed during 1974–1975 in three habitats in Cameroon. Observations took place during the wet and dry seasons in sahelian savannah, guinea savannah and farmed forest. In the sahel, large groups did not appear to defend territories. In the other two habitats, smaller groups did so. The different patterns of territoriality and group size appeared to be the result of differences in food supply and predation pressure