The noble scallop Chlamys nobilis has been an important marine cultured bivalve in the Southern Sea of China for decades. However, large-scale mortality events often occurred during the scallop' cultivation. As one of AMPs (antimicrobial peptides), big defensin is an important component of the innate immunity against pathogenic microorganisms in invertebrates. In order to investigate whether the big defensin can play a role in the immune defense against pathogenic microorganisms in noble scallop, a big defensin gene from the hemocytes of Chlamys nobilis (CnBD) was cloned, and the mRNA level was measured after an acute Vibrio parahaemolyticus challenge of 36 h. The CnBD cDNA contains an open reading frame (ORF) of 381 bp encoding a peptide of 126 amino acids residues. The deduce amino acid sequence of CnBD shows a high similarity with that from Argopecten irradians and displays common features of big defensin, indicating that CnBD is a new member of the big defensin family. Compared with the control group, the relative mRNA level of CnBD was significantly up-regulated at 3, 24 and 36 h. The present result indicated that CnBD played an immune role against bacterial infection in noble scallop.