The complete primary structure of histone H2A from the nematode Caenorhabditis elegans was determined. The amino acid chain consists of 126 amino acid residues and has a blocked N-terminus. By comparison with calf thymus histone H2A, the nematode protein shows five deletions, two insertions and 16 substitutions. Most of the changes occur in the N- and C-terminal regions of the molecule, whereas the central part covering the residues 21-120 is quite well conserved. The lysine residues 5, 8 and 10 were found to be partially acetylated.