Genetic studies in Drosophila and in vertebrates have implicated basic helix-loop-helix (bHLH) genes in neuronal fate determination and cell type specification. We have compared directly the expression of Mash1 and neurogenin1 (ngn1), two bHLH genes that are expressed specifically at early stages of neurogenesis. In the PNS these genes are expressed in complementary autonomic and sensory lineages. In the CNS in situ hybridization to serial sections and double-labeling experiments indicate that Mash1 and ngn1 are expressed in adjacent and nonoverlapping regions of the neuroepithelium that correspond to future functionally distinct areas of the brain. We also showed that in the PNS several other bHLH genes exhibit similar lineal restriction, as do ngn1 and Mash1, suggesting that complementary cascades of bHLH factors are involved in PNS development. Finally, we found that there is a close association between expression of ngn1 and Mash1 and that of two Notch ligands. These observations suggest a basic plan for vertebrate neurogenesis whereby regionalization of the neuroepithelium is followed by activation of a relatively small number of bHLH genes, which are used repeatedly in complementary domains to promote neural determination and differentiation.