The mec-3 gene is essential for proper differentiation of the set of six touch receptor neurons in C. elegans. In mutants lacking mec-3 activity, the touch receptors express none of their unique differentiated features and appear to be transformed into other types of neurons. We cloned the mec-3 gene by transposon tagging and showed that a mec-3 mutant can be rescued by germ line transformation using a 5.6 kb genomic DNA fragment. In a strain in which transforming mec-3 DNA is present in about 50 copies per haploid genome, additional cells express a mec-3-dependent phenotype. The putative coding sequence of mec-3 contains a homeobox, suggesting that the mec-3 protein specifies the expression of touch cell differentiation by binding to DNA and regulating transcription of genes that encode the differentiated features of these cells.