The black-faced black spider monkey, Ateles paniscus chamek, was studied at Cocha Cashu, Manu National Park, Peru, from June to August 1982. The density of independently locomoting individuals was found to be 31/km2, and the average party size was 3.15. Data on age and sex compositions of parties, activity patterns, and diet composition are presented. The spider monkeys spend approximately 30% of observed time feeding, 44% resting, and 25% moving. They ate 80% fruit and 17% new leaves. Spider monkeys appear to be important seed dispersers. The best dispersal observed was for fruits with few, relatively large seeds. A rough day-range of 2,400 m was estimated from measured travel times and distances. The social system of Ateles is discussed.