The small didelphid cmarsupial, Monodelphis domestica, uses a lateral sequence walk during slow treadmill locomotion and gradually shifts to a trot as speed increases. At higher speeds it changes abruptly to a half-bound. Cinematographic records suggest significant lateral bending but no sagittal bending of the trunk during the slow walk and a reduced amount of lateral bending during the fast walk. There is slight lteral, but no sagittal, bending during the trot. Sagittal bending is obvious during the half-bound, but no lateral bending is evident. Cineradiography confirms that the vertebral column of the trunk bends laterally during the slow walk. Bending occurs throughout the trunk region, but seems to be most pronounced in the anterior lumbar region. Associated with this bending of the trunk is substantial rotation of the pelvic girdle in the plane of yaw. Pelvic rotation is synchronized with the locomotor cycle of hindlimbs. Each side of the pelvis rotates forward during the recovery phase of the ipsilateral hindlimb and backward during the contact phase of this limb. Information on locomotor trunk movements in other limbed tetrapods is limited. The pattern of trunk bending found in Monodelphis, however, is consistent with that reported in the placental mammal Felis catus and in some lepidosaurian reptiles. This suggests that sagittal bending did not replace lateral bending during the evolution of mammals, as is sometimes suggested. Rather, bending in the vertical plane seems to have been added to lateral bleeding when the ancestors of extant mammals acquired galloping and bounding capabilities.