Abstract: Catalase (CAT) is the main member of antioxidant enzyme system, which plays an important role in maintaining redox homeostasis and resisting pathogen infection. In order to study the function of CAT gene in mollusc under pathogen infection, the full-length cDNA sequence of CAT in Sinonovacula constricta was cloned by RACE approaches, and designated as ScCAT. The full-length cDNA of ScCAT was 2840 bp and encoded a polypeptide of 508 amino acid residues. Sequence analysis showed that ScCAT protein contains a CAT core domain (25-410), a catalase proximal active site signature (₆₁FNRERIPERVVHAKGAGA₇₈) and a proximal heme-ligand signature sequence (₃₅₁RLFSYPDTH₃₅₉). Multiple sequence alignment and phylogenetic tree analysis confirm that ScCAT belongs to the CAT family, and is closer to invertebrate Meretrix meretrix. Tissue distribution analysis revealed that ScCAT was constitutively expressed in all examined tissues, and the highest expression was found in the hepatopancreas (85.67-fold, P < 0.01), followed by gill (50.09-fold, P < 0.01), and the lowest level was detected in hemocytes (0.76-fold) compared to that of adductor muscle. After the razor clams were challenged by Vibrio parahaemolyticus, the mRNA level of ScCAT was significantly increased in the hepatopancreas, and reached the highest level at 12 h compared with control (3.56-fold, P < 0.01). Moreover, the ScCAT protein activity in hepatopancreas and gills were significantly increased after V. parahaemolyticus challenge, with the higher magnitude in hepatopancreas. In addition, the recombinant protein was expressed in Escherichia coli, and the purified ScCAT showed highly catalase activity. All these results show that ScCAT is an important antioxidant enzyme, which participats in the immune response of S. constricta.