The stereotyped distribution of identified neurons and glial cells in the leech nervous system is the product of stereotyped cell migrations and rearrangements during embryogenesis. To examine the dependence of long-distance cell migrations on positional cues provided by other tissues, embryos of Theromyzon rude were examined for the effects of selective ablation of various embryonic cell lines on the migration and final distribution of neural and glial precursor cells descended from the bilaterally paired ectodermal cell lines designated q bandlets. The results suggest that neither the commitment of q-bandlet cells to migrate nor the general lateral-to-medial direction of their migration depend on interactions with any other cell line. However, the ability of the migrating cells to follow their normal pathways and to find their normal destinations does depend on interactions with cells of the mesodermal cell line, which appears to provide positional cues that specify the migration pathways.