Embryonic lethal mutations in the nematode Caenorhabditis elegans were generated and screened for phenotypes that suggest regulatory functions in order to identify genes involved in the control ofearly development. In embryos homozygous for mutations in one such gene, pha-1, the pharynx fails to undergo late differentiation and morphogenesis. Early pharynx development is not affected; thus, pha-l controls the latter stages of this developmental process. All markers specific for differentiation in various pharyngeal cell types tested are affected, suggesting that pha-1 acts in an organ-specific, rather than cell type-specific, manner. The temperature-sensitive phases of both temperature sensitive mutations indicate that pha-l function is required solely during midembryogenesis, shortly before the onset of morphogenesis.