The five main histones were isolated from mature sperm of the sea urchin Tetrapygus niger. The electrophoretic mobility and amino acid composition of these proteins were compared with data from the literature on histones from sperms of other species of sea urchins. Electrophoretic analysis revealed that H1, H2A, H2B and H3 migrate as a single protein; but different from other species, H4 was resolved into two variants. The amino acid composition of the nucleosomal core histones differed markedly from the analogous fractions of other species; the most important of these differences are a lower content in lysine and arginine and a higher amount of glutamic acid and serine residues. In contrast with the nucleosomal core histones, sperm H1 histone is highly conserved among the different species of sea urchins, suggesting that this histone might be involved in the maintenance of the higher order structure of sperm chromatin.