New World monkeys vary considerably in the manner and degree to which they use their tails. There ought to arise a corresponding variation in the stresses on caudal vertebrae. Consequently, different behavior types will have contrasting caudal morphologies. This study investigates caudal morphology of several New World monkeys representing the large range of ability from totally nonprehensile tails to the dexterous fifth limb of spider monkeys. One largebodied, nonprehensile-tailed Old World monkey is included to offset the correlation between size and degree of prehensility in New World monkeys. These monkeys vary both within and between species as to the number of caudal vertebrae and the length of individual vertebrae. Despite this variation, the length of caudal vertebrae within a given tail decreases at a constant rate across species. The width of the vertebrae at two points, midbone and the proximal end, separates the monkeys with prehensile tails from those with nonprehensile tails. The middle of the vertebrae is wider and more robust in prehensile-tailed forms. This is associated with stronger bones in tails subject to greater stress. The proximal width of each bone is also greater in the prehensile-tailed monkeys. This measure reflects the lateral development of the proximal transverse process. This provides a larger lever arm for M. caudalis lateralis, implying greater rotational ability.