The primary structure of a third H2B histone isolated from sperm of the sea urchin Parechinus angulosus has been determined. H2B(3) consists of a polypeptide chain of the following 148 amino acid residues: Pro-Arg-Ser-Pro-Ala-Lys-Thr-Ser-Pro-Arg-Lys-Gly-Ser-Pro-Arg-Lys-Gly-Ser-Pro-Arg-Lys-Gly-Ser-Pro-Ser-Arg-Lys-Ala-Ser-Pro-Lys-Arg-Gly-Gly-Lys-Gly-Ala-Lys-Arg-Ala-Gly-Lys-Gly-Gly-Arg-Arg-Arg-Arg-Val-Val-Lys-Arg-Arg-Arg-Arg-Arg-Arg-Glu-Ser-Tyr-Gly-Ile-Tyr-Ile-Tyr-Lys-Val-Leu-Lys-Gln-Val-His-Pro-Asp-Thr-Gly-Ile-Ser-Ser-Arg-Ala-Met-Ser-Val-Met-Asn-Ser-Phe-Val-Asn-Asp-Val-Phe-Glu-Arg-Ile-Ala-Ser-Glu-Ala-Ser-Arg-Leu-Thr-Ser-Ala-Asn-Arg-Arg-Ser-Thr-Val-Ser-Ser-Arg-Glu-Ile-gln-Thr-Ala-Val-Arg-Leu-Leu-Leu-Pro-Gly-Glu-Leu-Ala-Lys-His-Ala-Val-Ser-Glu-Gly-Thr-Lys-Ala-Val-Thr-Lys-Tyr-Thr-Thr-Ser-Arg. H2B(3) Parechinus closely resembles HIB(2) Parechinus but has one additional repeating pentapeptide in the amino-terminal region and a serine replacing glycine at position 98.