While the nematode Caenorhabditis elegans is more primitive than most egg-laying organisms, it's vitellogenins, or yolk protein precursors, appear to be more complex. C. elegans oocytes accumulate two major classes of yolk proteins. The first consists of two polypeptides with an M r of about 170,000 (yp170A and yp170B) encoded by a family of five closely related genes called vit-1 through vit-5. The second class consists of two smaller proteins with M r values of 115,000 (yp115) and 88,000 (yp88) which are cut from a single precursor. Here we report the cloning and analysis of a single-copy gene (vit-6) that encodes this precursor. The lengths of the gene and its mRNA are about 5× 10 3 base pairs. Like vit-1 through vit-5, vit-6 is expressed exclusively in adult hermaphrodites. Comparison of portions of the coding sequence indicates that vit-6 is distantly related to the vit-1 through vit-5 gene family. Thus, even though the two classes of yolk proteins are antigenically and physically distinct, they are encoded by a single highly diverged gene family.