In Caenorhabditis elegans the vitellogenin genes are expressed abundantly in the adult hermaphrodite intestine, but are otherwise silent. In order to begin to understand the mechanisms by which this developmental regulation occurs, we used the transformation procedure developed for C. elegans by A. Fire (EMBO. J., 1986, 5, 2673-2680) to obtain regulated expression of an introduced vitellogenin fusion gene. A plasmid with vit-2 upstream and coding sequences fused to coding and downstream sequences of vit-6 was injected into oocytes and stable transgenic strains were selected. We obtained seven independent strains, in which the plasmid DNA is integrated at a low copy number. All strains synthesize substantial amounts of a novel vitellogenin-like polypeptide of 155 kDa that accumulates in the intestine and pseudocoelom, but is not transported efficiently into oocytes. In two strains examined in detail the fusion gene is expressed with correct sex, tissue, and stage specificity. Thus we have demonstrated that the nematode transgenic system can give proper developmental expression of introduced genes and so can be used to identify DNA regulatory regions.