We have molecularly identified a new gene of Caenorhabditis elegans that codes for a component of the cuticle. The gene has been physically mapped on LGII near the locus sqt-1. The structure and the sequence of the gene have been determined and antisera have been raised against parts of the protein produced as fusions in Escherichia coli. By transcription analysis, and by the use of specific antisera, we have determined that this gene is expressed specifically during dauer larva formation. In extracts of worms completing the dauer transformation the product of this gene migrates in sodium dodecyl sulfate acrylamide gels with an apparent molecular mass of 40 kDa. By immunofluorescence we have determined that it is a component of the cuticles of dauer larvae. It forms a ribbon approximately 2 microns wide running along the lateral lines underneath the alae. Once it is assembled in the cuticle the protein becomes insoluble even in the presence of strong detergents and reducing agents in a manner that is similar to that described for the noncollagenous, insoluble residue of nematode cuticles called cuticlins; therefore, we have named the gene cut-1 for cuticlin 1. cut-1 represents the first gene for a noncollagenous component of C. elegans cuticle that has been characterized molecularly.