THE basic-helix-loop-helix (bHLH) proteins constitute a class of transcription factors thought to be important in the control of cell-type determination(1). These transcription factors are believed to activate the expression of cell-type-specific genes to generate stable differentiated cell types(2). The expression of bHLH proteins, in turn, is regulated by spatial cues, so that switches in cell type occur in a reproducible pattern(3). We report here that the lin-32 gene of Caenorhabditis elegans, which encodes a bHLH protein of the Drosophila achaete-scute family of transcription factors, is necessary and in some cells sufficient for specification of the neuroblast cell fate. Similarity in the function and structure of the lin-32 protein (LIN-32) to transcription factors of the achaete-scute gene family in Drosophila and vertebrates implies that this class of transcription factors functioned in a primitive ancestral form to specify neuronal cell fate, supporting the proposition that certain basic mechanisms of cell-type determination have been conserved through metazoan evolution(1).