Abstract: Catalase(EC 1.11.1.6) is an important antioxidant enzyme that protects aerobic organisms against oxidative damage by degrading hydrogen peroxide to water and oxygen. In the present study, the complete cDNA of Catalase in ark shell Scapharca broughtonii(named SbCAT) was cloned by RT-PCR and rapid amplification of cDNA ends technique, which contained 2181 bp. The cDNA consists of a 5' untranslated region(UTR) of 96 nucleotides, the 3' UTR of 654 bp, and an open reading frame(ORF) of 1431 bp, encoding 477 amino acid residues with 54 ku predicted molecular weight and the theoretical isoelectric point of 8.03. SbCAT amino acid sequence compared with the other animals compared 68%-96%.The deduced amino acid sequence of SbCAT has characteristic features of catalase family such as the catalase active site(₆₁FNRERIPERVV-HAKGAG₇₇), the catalase heme-ligand signature motif(₃₅₁RLFSYPDTH₃₅₉) and the three catalytic amino acid residues of His-72, Asn-145 and Tyr-355. In addition, SbCAT also has the conservative heme-binding pocket and NADPH binding sites. Quantitative real-time PCR(qRT-PCR) method was used to analyze the SbCAT mRNA expression characterization in tissues of normal ark shells. The results showed that SbCAT mRNA detected was expressed in six had a high similarity of and it was higher in the mantle and lower in the tissues of hepatopancreas and haemocyte. After Vibrio anguillarum and Staphylococcus aureus challenge, SbCAT expression was rather low in mantle with V.anguillarum challenged, and significantly up-regulated in other tissues. This results suggests that SbCAT may play an important role in the immune defense of S. broughtonii.