The blood clam Barbatia virescens expresses a unique heterodimeric hemoglobin consisting of chains I and II in erythrocytes. This is in sharp contrast to the tetrameric (α₂β₂) and polymeric two-domain hemoglobins of the congeneric species Barbatia reeveana and Barbatia lima. The 3′ and 5′ parts of the cDNA of B. virescens chain II have been amplified separately by polymerase chain reaction (PCR), and the complete nucleotide sequence of 690 bp was determined. The open reading frame is 477 nucleotides in length and encodes a protein with 158 amino acid residues, of which 120 amino acids were identified directly by the protein sequencing of the peptides obtained from digestions with trypsin, S. aureus V8 protease and pepsin. The mature protein begins with the blocked Ser, and thus the N-terminal Met is cleaved away. The molecular mass for the protein was calculated to be 17605 Da. The cDNA-derived amino acid sequence of B. virescens heterodimeric chain II shows the highest homology (42%) with that of B. virescens chain I, but shows lower homology (32–35%) with those of tetrameric α and β chains of B. lima. This indicates that B. virescens chains I and II do not correspond to B. lima α and β chains, namely the heterodimeric hemoglobin is a unique gene product expressed only in B. virecens.