The nematode Caenorhabditis elegans expresses one species of H2A and one species of H4 molecules, at least two species of H1 (H1.1, H1.2), two species of H2B (H2B.1, H2B.2) and 2-4 species of H3 (H3.1 and H3.3 and an unassigned Ile/Leu microheterogeneity in H3). The study of their primary structures has been completed now and all of them, with the exception of the Ile/Leu microheterogeneity in H3, have been assigned to protein spots on two-dimensional gels. One spot, previously designated H3.2, probably represents C-terminally cleaved H3.1. The relative abundance of the isohistones was essentially the same when derived from either eggs, gravid adults or postreproductive, senescent worms. The degree of post-translational modification, however, particularly acetylation of H2A, H2B and H3 histone species, was reduced at old age.