Abstract: The lipopolysaccharide ( LPS) was isolated from the cell wall of a bacterial pathogen Pseudomonas maltophilia which was separated from Trachinotus ovatus cultured in Hainan Province. And then, it was immunized to healthy Trachinotus ovatus by the single i. p. injection, double i. p. injection and oral administration, respectively . The result s showed that, w ithin 2- 12 weeks after the f ish was inoculated with LPS, its agglutinating antibody titers against dead Pseudomonas maltophilia and its resistance to the bacterial pathogen Pseudomonas maltophilia increased signif icant ly. Moreover, the phagocytic act ivity of the leucocytes of Trachinotus ovatus also increased after it was immunized with LPS prepared from Pseudomonas maltophilia.