This monograph presents a detailed report on the social behaviour and ecology of a previously little known species, the capped langur (presbytis pileata). The author observed capped langurs at Madhupur National Park in Bangladesh, where they live in small one-male groups. This account of his 15-month field study focuses on the relationships among one-male groups and the influence of ecology and seasonal variation in diet upon social behaviour. Collected field data is presented on diet, ranging, leadership, activity patterns and aspects of intra-group and intergroup behaviour, whch are then used to test predictions of traditional ecological models of primate social systems. Features of the capped langur's behaviour and ecology that differ from previously studied colobine monkeys are also described in detail.