An intracellular copper/zinc superoxide dismutase (cpSOD) cDNA was cloned from the haemocytes of freshwater mussel Cristaria plicata using reverse transcription-polymerase chain reaction (RT-PCR) by degenerate primers.Further studies were conducted on cpSOD mRNA expression in various tissues of C.plicata by semi-quantitative PCR method.The results revealed that the cpSOD cDNA clone consists of 891 bp with an open reading frame of 468 bp and with a protein of 155 amino acids.The calculated molecular mass is 15.8 ku with an estimated pI of 5.8.Four conserved amino acids responsible for binding copper (H46,H48,H63 and H120),four conserved amino acids responsible for binding zinc (H63,H71,H80 and D83) and a conserved intramolecular disulfide bond were observed.Sequence comparison showed that the deduced amino acid sequence of cpSOD presented 70% similarity to that of saltwater shellfish Crassostrea gigas ic-Cu/Zn-SODand 69% to Chlamys farreri ic-Cu/Zn-SOD,respectively.Semi-quantitative RT-PCR analyses showed that cpSOD genes were expressed in haemocytes,mantle,gill,adductor muscle and hepatopancreas of C.plicata.Stimulating C.plicata by injection with Aeromonas hydrophila significantly up-regulated the expression of cpSOD in haemocytes,indicating that cpSOD is involved in the immune responses against A.hydrophila.