Aeromonas hydrophila is a fish pathogen causing systemic infections in aquatic environments, and determining its antigenic proteins is important for vaccine development to reduce economic losses in aquaculture worldwide. Here, an immunoproteomic approach was used to identify immunogenic outer membrane proteins (OMPs) of the Chinese vaccine strain J-1 using convalescent sera from Chinese breams. Seven unique immunogenic proteins were identified by two-dimensional (2-D) electrophoresis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-TOF-MS). One protein of interest, Omp38, was expressed, and its immunogenicity and protective efficacy were evaluated in Chinese breams. The two groups of fish immunized with the inactivated vaccine and recombinant Omp38 protein showed significant serum IgM antibody levels after vaccination, compared with the fish injected with PBS buffer. In addition, the superoxide dismutase (SOD) activity, lysozyme (LSZ) activity and phagocytosis activity of head kidney lymphocytes of immunized groups were significantly higher than those of the control. The fish receiving inactivated vaccine and recombinant Omp38 protein developed a protective response to a live A. hydrophila challenge 45 days post-immunization, as demonstrated by increased survival of vaccinated fish over the control and by decreased histological alterations in vaccinated fish. Furthermore, protective effect was better in Omp38 group than in the inactivated vaccine group. These results suggest that the recombinant Omp38 protein could effectively stimulate both specific and non-specific immune responses and protect against A. hydrophila infection. Therefore, Omp38 may be developed as a potential vaccine candidate against A. hydrophila infection. Fish & Shellfish Immunology. January201334(1):74-81. Na WangZhao YangMingfa ZangYongjie LiuChengping Lu. Key Lab of Animal Bacteriology, Ministry of Agriculture, Nanjing Agricultural University, No. 1 Weigang, Nanjing 210095, China.