The respiratory proteins of the invertebrate marine mollusk A. trapezia were separated by a combination of gel filtration and ion-exchange chromatography. There is a major tetrameric Hb, HbI, of MW approximately 67,000 and 2 minor polymorphic HbIIa and HbIIb that are dimeric with MW approximately 31,000, in agreement with previous reports in the literature. The tetrameric Hb contained .alpha.- and .beta.-globin chains that were separated by ion-exchange chromatography in 8 M urea-thiol buffers. The amino acid compositions of the chains showed considerable differences. The amino-terminal residues of these globin chains appear to be blocked, but C-terminal residues of leucine and alanine were released by carboxypeptidase A digestion. Both HbIIa and HbIIb contain identical subunits with prolyl amino-terminal residues; alanine appeared to be the C-terminal residue from the rate of liberation by carboxypeptidase A digestion. The amino acid compositions for the 2 polymorphic globins showed differences only in lysine, aspartic acid and glycine. The N-terminal sequence of both HbIIa and HbIIb globin chains was Pro-Ser-Val-Glu-Asp-Ala-Ala-Thr-Leu-Lys.