The molecular components of the surface of the free-living nematode Caenorhabditis elegans have been identified by surface-specific radioiodination. Four compartments were defined by fractionation of labeled wild type (N2 strain) adult hermaphrodites. Organic solvents extracted cuticular lipids. Homogenization in detergents released a single, non-collagenous, hydrophobic protein. This is not glycosylated and is a heterodimer of 6.5- and 12-kDa subunits. The third compartment, proteins solubilized by reducing agents, included both the cuticular collagens and the heterodimer. Residual material corresponds to the cuticlin fraction. Larval stages showed a similar pattern, except that the dauer larva had an additional 37-kDa detergent-soluble protein. Other species of rhabditid nematodes displayed similar profiles, and comparison with parasitic species suggests that this simple pattern may be primitive in the Nematoda. A C. elegans strain mutant in cuticular collagen (rol-6) had a pattern identical to that of wild type, but another morphological mutant (dpy-3) [corrected] and several mutants that differ in surface reactivity to antibody and lectins (srf mutants) also had striking differences in surface labeling patterns.