The researches on antibacterial peptides from Mytilus coruscus, an important mytilus on aquiculture, have significant value that helping people to understand the mechanism of innate immune system of this mussel. By using multi-dimensional reverse phase high performance liquid chromatography (RP-HPLC), a novel antimicrobial peptide was isolated from Mytilus coruscus serum. The mass and the N-terminal sequence of this peptide were analyzed by a combination of Mass Spectrometry and Edman degradation. This peptide consisted 55 amino acids and had a 6621.55 D with 6 Cysteines presented in its sequence forming 3 disulfide bonds. A chitin-biding domain was detected by domain analysis and the peptide was named mytichitin-A therefore. The mRNA transcripts of mytichitin-A were mainly detected in gonad, which indicated that mytichitin-A were specifically synthesized and stored in genital system. The expression level of mytichitin-A in gonad was up-regulated and reached to the highest level at 12h after bacterial challenge, which was 9-fold increase compared to that of the control group. These results indicated that mytichitin-A was involved in the host immune response against bacterial infection and might contribute to the clearance of invading bacteria.