The embryonic lethal gene pha-1 of the nematode Caenorhabditis elegans is required for late differentiation and morphogenesis of the pharynx in the developing embryo. Revertants of two temperature-sensitive alleles of pha-1 were isolated with the aim of obtaining mutations in genes that interact with pha-1. By various methods of mutagenesis, chemical, X-ray, transposon, or by spontaneous reversion, 220 recessive revertants were obtained, defining three complementation groups. The largest, sup-35 on linkage group (LG) III, maps close to but is separable from pha-1. This suppressor can exert its effect either maternally or zygotically to allow survival of pha-1(ts) embryos. The other two, sup-36 and sup-37, are required zygotically and map on LGIV and LGV, respectively. We have not noted a phenotype distinguishing any of the suppressors from wild type except for suppression of pha-1. That suppression is the null phenotype of at least sup-35 is indicated by the high frequency of mutation and by the fact that heterozygotes carrying sup-35 and a deficiency spanning the locus are also able to suppress. Five spontaneous mutations in sup-35 were found to be associated with recombination.