Leiognathid fish (Perciformes: Leiognathidae) contain 17 species in Taiwan in three genera: Gaza, Leiognathus, and Secutor. Leiognathid fish form mutualistic bio- luminescent symbiosis with the luminous bacteria Photobacterium leiognathi and have developed a specialized light organ to harbor bacteria and other structures to control light emission. The ecological dependence of leiognathid fish to its symbiont for light production and the specialized adaptations suggest that there co-evolution might have taken place. A single symbiont species were thought to be harbored in the light organ of leiognathid fishes. However, different oxygen rates, light production, and cell morphology have been observed in bacteria culturing during previous studies. To investigate the co-evolutionary interaction between P. leiognathi and leiognathid species, I examined the 2-DE patterns of the outer membrane protein (OMP) of symbiotic bacteria from specimens of five leiognathid species; OMPs have been proposed to be essential in symbiotic interaction. The 2-DE results of the P. leiognathi OMP pattern from five leiognathid species displayed intra-specific similarity, when inter-specific differences also exist. Intra-specific consistence revealed five proteins that are essential in the symbiotic interaction. Spot differences between various leiognathid species have shown that closely related species have significant protein spots which may have effect on speciation. In Secutor ruconius absence of the protein responsible for motility might result the difficulty on bacteria culturing. Intra-specific similarity excludes the possible of individual differentiation, and the inter-specific differences of bacterial OMP suggested that P. leiognathi in various leiognathid species might have developed various OMPs to adapt to different host species.