Neural crest cells migrate during embryogenesis to give rise to segmented structures of the vertebrate peripheral nervous system: namely, the dorsal root ganglia and the sympathetic chain. However, neural crest cells arise from the dorsal neural tube where they are apparently unsegmented. It is generally agreed that the somites impose segmentation on migrating crest cells, but there is a disagreement about two basic questions:(1) exactly what pathways do neural crest cells use to move through or around somites, and (2) do neural crest cells actively migrate or are they passively dispersed by the movement of somite cells? The answers to both questions are critically important to any further understanding of the mechanisms underlying the precise distribution of the neural crest cells that develop into ganglia. We have done an exhaustive study of the locations of neural crest cells in chick embryos during early stages of their movement, using antibodies to neural crest cells (HNK-1), to neural filament-associated protein in growing nerve processes (E C 8), and to the extracellular matrix molecule laminin. Our results show that (1) Some neural crest cells invade the extracellular space between adjacent somites, but the apparent majority move into the somites themselves along the border between the dermatome/myotome (DM) and the sclerotome.(2) Neural crest cells remain closely associated with the anterior half of the DM of developing somites as they travel, suggesting that the basal lamina of the DM may be used as a migratory substratum. Supporting this idea is our observation that the development of the DM basal lamina coincides in time and location with the onset of crest migration through the somite.(3) The leading front of neural crest cells advance through the somite while the length of the DM pathway remains constant, suggesting active locomotion, at least in this early phase of development.(4) Neural crest cells leave the DM at a later stage of development to associate with the dorsal aorta, where sympathetic ganglia form, and to associate with newly emerging fibers of the ventral root nerve, where they presumably give rise to neuronal supportive cells. Thus we propose that the establishment of the segmental pattern of the peripheral ganglia and nerves depends on the timely development of appropriate substrata to guide and distribute migrating neural crest cells during the early stages of embryogenesis.