Abstract: We conducted examinations to seven cases of flounder infected by Edwardsiella tarda, and observed the situation of disease, clinical symptoms and pathological changes.Theresults showed that all the cases were infected only by Edwardsiella tarda through isolation and identification to pathogenicbacteria. The authors summarized themain traits of this infection. In addition,the biological characteristics and serotype of 130 strains pure cultures of Edwardsiella tarda were detected, the results revealed that the biochemical characteristics were all common in these strains except for the test of indolewhich occur difference between strains (20 strains were negative, 110 strains were positive), and all these strains belonged to the same serotype. Every one repre-sentative strains isolated from these infections were proved to be the corresponding primitive causal agent of the disease and strong pathogenicity by artificial infection experiment to healthy flounder. Antimicrobial susceptibility tests showed that all strains were sensitive to cefazolin et al.,were resistant to penicillin G et al., and existed difference between strains to ampicillin et al. in the used thirty-seven drugs.Through examination to 130 strains of pure cultures and isolates from liver of moribund fish from infection experiment by fluorescent antibody technique, showed the feasibility of fluorescent antibody technique as an auxiliary method in the examination of Edwardsiella tarda.