Abstract: C9, a component of the membrane attack complex, participates in the final stage of the complement cascade which lyses foreign organisms by disrupting the integrity of their cell membranes. In the present study, a full-length ayu C9 (aC9) cDNA was cloned which contains 2,125 nucleotides and encodes a protein of 592 amino acids.A signal peptide was deposited in the N-terminal 22 residues. The deduced amino acid sequence of aC9 showed 56.8% identity to the C9 of rainbow trout, and 40.9% to 53.8% identity to the C9 of other teleosts. RT-PCR analysis demonstrated that the mRNA of aC9 was expressed in the liver, spleen, intestine, gill and muscle of healthy ayu fish with the highest level in the liver. Quantitative RT-PCR analysis showed that aC9 transcripts were significantly up-regulated in the liver at 4 h post Listonella anguillarum infection, peaked at 16 h post injection. Western blotting analysis revealed that serum aC9 significantly increased in Listonella anguillarum infected ayu fish. Our results suggested that aC9 may play an important role in fish immune response of anti-bacteria.