The recent production of a monoclonal antibody (NC-1) recognizing migrating avian neural crest (NC) cells (M. Vincent, J. L. Duband , and J. P. Thiery , Dev. Brain Res. 9, 235-238, 1983) allowed us to detail their migration pathways at the trunk level of the chick embryo. Three routes can be recognized: NC cells facing the bulk of the somite accumulate to form a spinal ganglion, those facing the intersomitic space can readily reach periaortic areas to contribute to the primary sympathetic chain, and cells at intermediate levels between these two accumulate between the neural tube and the somite but some of them can escape between the sclerotome and the myotome and settle near the aorta. Histological and in vitro immunofluorescence patterns have demonstrated that the NC-1 antigen is a neuroectodermal feature. In addition to its presence on the great majority of NC cells, it persists at the surface of both neuronal and satellite cells of the peripheral ganglia. Moreover, it can be detected on neurogenic placodes and their derivatives. The appearance of the NC-1 antigen in the central nervous system coincides with the first noticeable morphological changes of the neutral tube and develops according to a rostro-caudal gradient which parallels its development: it seems, however, to be transiently expressed by the neuron cell bodies and to concentrate later on their processes. It is also present on non-neuronal cells derived from the neuroectoderm. The neuroectodermal character of NC-1 reactivity is further emphasized by its disappearance from the melanocytes and the mesectodermal derivatives of the NC. The loss by the latter, in ventral areas of the head, of the NC-1 epitope is discussed in relation to previous findings on the degree of commitment of the cephalic NC. The NC-1 epitope is associated with several high-molecular-weight polypeptides and may involve a carbohydrate moiety.