The par-2 gene of Caenorhabditis elegans functions in early embryogenesis to ensure an asymmetric first cleavage and the segregation of cytoplasmic factors. Both processes appear to be required to generate daughter blastomeres with distinct developmental potential. We isolated an allele of par-2 by using a screen for maternal-effect lethal mutations in a strain known for its high frequency of transposition events. A transposable element was found to be linked to this allele. Sequences flanking the site of transposon insertion were cloned and found to rescue the par-2 mutant phenotype. DNA in the par-2 region hybridized to a 2.3-kb germ-line-enriched mRNA. The cDNA corresponding to this germ-line-enriched message was cloned, sequenced, and used to identify the molecular lesions associated with three par-2 alleles. Sequence analysis of the par-2 cDNA revealed that the predicted protein contained two distinct motifs found in other known proteins: an ATP-binding site and a cysteine-rich motif which identifies the par-2 gene product as a member of a growing class of putative zinc-binding proteins.