Found in only one small area in north-east Kenya, the Tana mangabey is both seriously depleted and highly endangered. Numbers are estimated at under 1500. The author, who spent two years on a field study in the Tana River area, shows how the combination of increased pressure from the growing human population and the long-term effects of new hydroelectric and irrigation schemes will affect especially the food supply of this monkey, which has adapted itself to a complicated river regime that may now be destroyed.