The forelimb anatomy of Ateles, the spider monkey, is a key factor in its locomotor adaptation because of its extensive use of bimanual suspensory locomotion in addition to climbing and quadrupedal walking (Mittermeier, 1978; and Fleagle and Mittermeier, 1980). The detailed description of the muscles and ligaments of the forelimb of Ateles provided in this paper can be of considerable use not only to primate morphologists interested in both extant and fossil primates but also to functional anatomists. All measurements were made on fresh, unembalmed specimens. Standard anatomical descriptions of the musculature, cross-sectional areas of the muscles, dry weights of muscle bellies, and relative proportions of antagonistic muscles are reported as are the joint ligaments and potential range of motions of the joints. These anatomical data are compared briefly to published data on Alouatta, the howler monkey, which is predominantly a quadruped (Fleagle and Mittermeier, 1980; and Schön Ybarra, 1982).