Sympatric Malaysian leaf-monkeys, Presbytis obscura and Presbytis melalophos show quantitative differences in their use of different locomotor patterns during daily activities. P. obscura primarily moves quadrupedally along large boughs; P. melalophos relies more on leaping between smaller supports. P. melalophos also uses forelimb suspension more than does P. obscura. Comparison of the muscular anatomy of the shoulder, arm, trunk, hip and thigh show numerous statistically significant differences between the two species in relative mass of individual muscles and in details of muscular origin and insertion. In almost all instances these differences accord with the types of differences in muscular anatomy predictable from either biomechanical analyses of locomotor behavior or comparative anatomical studies of other mammalian species.