In order to find out which muscles are involved in the respiratory component of primate phonation, the activity of 17 abdominal and thoracic muscles was recorded during vocalization in the squirrel monkey. Vocalization-correlated activity was found in the musculi obliquus externus et internus, rectus et transversus abdominis, intercostalis externus et internus and intercartilagineus. It was lacking in the mm. iliocostalis, latissimus dorsi, longissimus dorsi rhomboideus, serratus posterior superior, trapezius, splenius capitis, sternocleidomastoideus, scalenus medius and pectoralis major. There was simultaneous activation of the rib-raising external and rib-lowering internal intercostal muscles during most vocalizations. It is hence concluded that the intercostals, rather than supporting expiratory efforts, serve to stabilize the thorax, thus providing an anchorage against which the abdominal muscles can act.