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Lysozyme is a key component of the innate immune system, which plays a pivotal role in early defense against pathogen infection. In this study, an i-type lysozyme homology was identified from the razor clam Sinonovacula constricta (designated as ScLYZ) through RACE approaches. The full-length cDNA of ScLYZ was 768 bp and encoded a polypeptide of 140 amino acid residues. SMART analysis revealed that ScLYZ processed a signal peptide (1–18 aa) and a destabilase domain from 25 to 133 aa. Two catalytic residues (Glu36 and Asp47) and two specific motifs [“CL(E/L/R/H)C(I/M)C” and “MDVGSLSCG(P/Y) (F/Y)QIK”] of the i-type lysozyme were highly conserved in the ScLYZ sequence. Multiple sequence alignments and phylogenetic analysis indicated that ScLYZ could be a new member of the i-type lysozyme subfamily. Tissue distribution analysis revealed that ScLYZ was constitutively expressed in all examined tissues, and the highest expression was found in the hepatopancreas. After the razor clams were challenged by Vibrio parahaemolyticus, the mRNA levels of ScLYZ increased in the gill and hepatopancreas. Moreover, the recombinant protein was expressed in Escherichia coli, and the refolded ScLYZ showed highly antimicrobial activities against V. parahaemolyticus and Vibrio splendidus. The minimal inhibitory concentration toward V. parahaemolyticus was 8.2μmol/mL. All our results supported that ScLYZ was involved in the innate immune defense of razor clam by inhibiting the growth of invasive pathogens.A novel gene of lysozyme was identified in Sinonovacula constricta and its transcript was ubiquitously expressed in all examined tissues.Vibrio parahaemolyticus could significantly up-regulate the expressions of ScLYZ both at the tissues of gill and hepatopancreas.The refolded ScLYZ protein showed highly antimicrobial activities against V. parahaemolyticus and Vibrio splendidus, and the minimal inhibitory concentrations (MIC) toward V. parahaemolyticus at a concentration of 8.2μmol/mL.