One strain of bacteria 0251721 was isolated from Lateolabrax japonicus with the bacterial pseudotuberculosis in May 2002. After the pure culture , the bacteria were used to infect the health Lateolabrax japonicus in three ways : abdominal cavity injection ( 0. 3mL/ nos ) , wound soaking , and soaking. The concentration of bacterial suspension was determined by both McF and live bacterial counting technique. The physiological saline was substitute for bacterial suspension in the control. The concentration of soaking was 1.52 ?104 Cells ?mL -1. The result showed that the strain of bacteria 0251721 had powerful pathogenicity and the mortality of healthy fish was 100 %. According to Bergey’s , manual of systematic bacteriology and API 20E system indentification , the pathogen of Lateolabrax japonicus was identified as Aeromonas hydrophila. Serums were extracted from the blood of both diseased fish and healthy fish , and the blood biochemical and physiological indices were determined by Olympus biochemical automatic instrument. The results showed that , compared with healthy ones , the number of red blood cells , white blood cells , glucose ,total plasma protein , ureanitrogen and cholesterinindiseasedfishalldecreasedremarkably,whileserumenzymesofALT,AST,LDHincreased. These results indicated that many important physiological functions such as digestion , breath , excretion , metabolism and disease resistance were seriously impaired after Lateolabrax japonicus infection. The result of antibiotic sensitivity test showed that four chemotherapeutants (compound sulfamethoxazole , kanamycin , gentamycinum , norfloxacin) and four Chinese herbal medicines (sanguisorba , armeniaca , chaenomeles , schisandra ) were most effective against the pathogenic bacteria.