The serratus ventralis in mammals is a fan-shaped scapulo-thoracic muscle that is believed by most morphologists both to support body weight and to rotate the scapula during quadrupedal locomotion. Electromyographic studies of this muscle in cats, dogs and opossums confirm the dual supportive and rotatory roles of the serratus ventralis. Although this muscle has been studied in several primate species, the concentration on arboreal locomotion has resulted in an inadequate data set to permit direct comparisons to non-primate terrestrial quadrupeds. In order to provide a more comparable data set, we examined cranial, mid- and caudal thoracic regions of the serratus ventralis during terrestrial quadrupedalism in the vervet monkey, Cereopithecus aethiops. Our results indicate that the serratus ventralis does support the body during the stance phase of quadrupedalism in this primate. However, unlike several non-primate mammals, it plays a relatively insignificant rotatory role during swing phase.