We report the cloning and sequencing of fem-1, a gene required for sex determination in both germline and somatic tissues in the nematode C. elegans. Clones carrying a 5.5 kb fragment are able to rescue the progeny of a fem-1 mutant when injected into its oocytes. The major fem-1 transcript in both sexes is 2.4 kb and comprises 11 exons. It encodes a soluble, intracellular protein of 656 amino acids that includes near its N-terminus six contiguous copies of a motif found in the products of the cdc10 gene of S. pombe, the SWI6 gene of S. cerevisiae, the Notch gene of Drosophila, and the lin-12 and glp-1 genes of C. elegans.