We describe interactions between maternal-effect lethal mutations in four genes of Caenorhabditis elegans whose products appear to be involved in the meiotic and mitotic divisions of the one-cell embryo. Mitosis is disrupted by two dominant temperature-sensitive gain-of-function maternal-effect lethal mutations,
mei-1(
ct46) and
mel-26(
ct61), and by recessive loss-of-function maternal-effect lethal mutations of
zyg-9. The phenotypic defects resulting from these mutations are similar. Doubly mutant combinations show a strong enhancement of the maternal-effect lethality under semipermissive conditions, suggesting that the mutant gene products interact. We isolated 15 dominant suppressors of the gain-of-function mutation
mei-1(
ct46). Thirteen of these suppressors are apparently intragenic, but 11 of them suppress in trans as well as cis. Two extragenic suppressors define a new gene,
mei-2. The suppressor mutations in these two genes also result in recessive maternal-effect lethality, but with meiotic rather than mitotic defects. Surprisingly, most of these suppressors are also able to suppress
mel-26(
ct61) in addition to
mei-1(
ct46). The products of the four genes
mei-1,
mei-2,
zyg-9 and
mel-26 could be responsible for some of the specialized features that distinguish the meiotic from the mitotic divisions in the one-cell embryo.