Cuticlin is the insoluble residue of nematode cuticle. It has been proved that cuticlin and CUT-1-like epitopes are conserved between the free-living Caenorhabditis elegans and the entomopathogenic nematode Heterorhabditis sp. The cloning of a cut-1 homologous gene from the animal intestinal parasite Ascaris lumbricoides has allowed us to extend the study of immuno-cross-reactivity at the ultrastructural level to this important species. Antibodies against recombinant CUT-1 protein and against cuticlin from Ascaris as well as from C. elegans were used for immuno-labeling ultrathin sections of high-pressure cryoprocessed worms. All the antisera used showed the same specific pattern of localization on sections of C. elegans of Heterorhabditis dauer larvae, and of Ascaris larvae in mature eggs. It was also shown that sera raised against the cuticlin residue contain anti-CUT-1 antibodies. CUT-1-like proteins are thus possibly important components in the immune response of hosts to invading nematodes. The results presented support the use of C. elegans as a model for the study of vertebrate parasitic nematodes.