±0.81) cm were temporally cultured for one week. We designed test trial and control trial with 20 individuals for each. Cristaria plicata. The full-length cDNA was 514 bp, consisting of a 5'-terminal untranslated region(UTR)of 57 bp, a 3'-terminal UTR of 242 bp with a poly(A)tail, and an open reading frame of 192 bp which encoded a polypeptide of 63 amino acids residues, including 23 signal peptides and 40 mature peptides. The predicted molecular weight was 7.13 kD, the theoretical isoelectric point was 8.72. The predicted structure revealed that defensin gene contained a conserved cysteine-stabilized α-helix and two β-sheet(CSαβ)structural motif. We measured the expression defensin in the hemocytes, mantle, gill, and hepatopancreas cell/mL). Before stimulation, the expression of defensin was highest in the mantle and was about 1.25 times that in hemocytes and gill. After injection(at 0, 6, 12, 24, 36, 48 h), the relative expression first decreased in the mantle and hepatopancreas then increased significantly in the control group. There was no change in defensin expression in the hemocytes and gill. In the experimental group, expression peaked after 12 h in the hemocytes then decreased. Conversely, defensin expression was highest in the other tissues at 0 h, and then decreased gradually. Expression increased slightly in the mantle and gill at 24 h and 36 h.