Gastrulation in Caenorhabditis elegans is normally initiated by inward migration of the two gut precursor (E) cells at the 26-cell stage. A strong loss-of-function, temperature-sensitive, embryonic lethal mutation in the maternally required gene gad-1 (gastrulation defective) prevents gastrulation initiation. In embryos from homozygous mutant gad-1 (ct226) hermaphrodites reared at 25 degrees C, the E cells divide early with abnormal spindle orientations and fail to migrate into the embryo, and no subsequent gastrulation movements occur. These embryos continue to develop and differentiate the major cell types, but they undergo little morphogenesis. The temperature-sensitive period of the mutant is during early embryogenesis, prior to gastrulation onset. The predicted translation product of the cloned gad-1 gene includes six beta-transducin-related repeats of the WD motif, which has been implicated in protein-protein interactions. The ct226 mutation alters a conserved residue in one of these repeats. Injection of gad-1 antisense RNA into wild-type hermaphrodites mimics the mutant phenotype in progeny embryos. We conclude that the gad-1 gene product is required for initiation of gastrulation in C. elegans.