Using the methods of muscle injection of horseradish peroxidase (HRP) and application of HRP to the proximal stump of the cut lateral thoracic nerve (LTN), the motor nucleus of the cutaneous trunci muscle (CTM) has been located predominantly in C8 and T1 spinal cord segments in the ventral and ventrolateral nuclei of Rexed's Lamina IX. Cells of the nucleus are somatotopically arranged rostro-caudally to correspond to the rostro-caudal segmentation. There was a higher average number of HRP-filled cells per section and greater rostro-caudal extent of the nucleus for the cut nerve technique than for the muscle injection technique. Evidence is presented that the CTM is an appendicular muscle in spite of its obvious axial position on the trunk.