One of strains were isolated from the liver of diseased soft-shelled turtle of furunculosis , red neck and red spot complicating diseases , which was proved to be the pathogen of the disease by the challenge test .The isolate was tested for more than 30 unit morpholohic , physiological and biochemistric characteristics , and classified as Aeromonas hydrophila .It was short straight rods (0 .36 ～ 0 .71μm×1 .0 ～ 1 .4μm), Gram negative ,single polar flagellum, neither capsule , nor spores but motible .ONPG test , arginine dihydrolase , indole production , Voges-proskauer reaction , gelatinase , oxidase and esculin were positive .Acid production from glucose ,mannitol , sucrose and Darabinose .Lysine decarboxylase , ornithine decarboxylase , utilization of sodium citrate , H2S production , urease and indolylpyruvic acid(IPA)were negative .The LD50 of the isolatod bacteria on Crucian carp is 9 .17 ×105CFU, and the LD50 on mice is 1 .59 ×107CFU .Optimum growth temperature , pH and salinity ranges from 25 ℃ to 35 ℃, 6 .5 to 7 .5 , and 5 to 15 sodium chloride , respectively . This isolate was highly sensitive to Ciprofloxacin , Ofloxacin , Amikacin , Chloramphenicol , Doxycycline , Tetracycline , Kanamycin , Aztreonam , Netilmicin , Cefuroxime , Piperacillin , Cefotaxime , Ceftriaxone and Ceftazidime , etc .