The glp-1 and lin-12 genes mediate several cell interactions during C. elegans development. We have identified the glp-1 gene in a region about 20 kb from lin-12. In collaboration with Yochem and Greenwald (1989; see accompanying paper), we show that a sequence identified by its similarity to lin-12 is in fact glp-1. We find a single 4.4 kb glp-1 transcript and a distinct 4.6 kb lin-12 transcript. Expression of the glp-1 transcript during development differs from that of lin-12. As expected from genetic analyses, glp-1 RNA is primarily in the germline while lin-12 RNA is primarily in the soma. Unexpectedly, we find that glp-1 RNA is also expressed in larval somatic tissues and that lin-12 RNA is abundant in early embryos. We suggest that glp-1 and lin-12 may play broader roles in development than previously thought.