Motoneuron pools supplying principal muscles of the shoulder girdle and wing were localized in the chicks 2-7 days post hatching with the use of retrograde axonic transport of horse radish peroxidase (HRP). HRP was injected into selected muscles and the animals sacrificed after 24 h survival. Labelled motoneuron pools representing individual muscles were found to be clustered in longitudinal columns along the brachial spinal cord segments 13-16. Muscles derived from the dorsal muscle mass were innervated by motoneurons located in the ventro-lateral portion of the horn, while those originating from the ventral muscle mass received innervation from neurons occupying the dorsomedial portion within the ventral horn of the spinal cord. The rostrocaudal extent of the motoneuron pools could be correlated with the proximodistal position of wing muscles. The observed orderly topographical relationships between clusters of motoneurons of the brachial spinal cord and the muscles they innervate will be used as baseline data for experiments where the limb innervation is perturbed.