Abstract: Taxonomic studies were performed on ninety-three bacterial isolates from nine species of fish affected by the septicemia and reared in fresh water in Hubei,Hunan, Henan and Guangdong during the period from May to October of 1990 and 1991. Analysing more than one hundred phenotypic characters and mol%G+C of the bacterial deoxyribonucleic acids, these isolates were assigned either to motile mesophilic aeromonad in the genus Aeromonas or to Vibrio fluvialis in the genus Vibrio. In conformity with the Priority of the International Code of Nomenclature of bacteria, these motile mesophilic aeromonads should be Aeromonas punctata(Zimmermann) nom. rev.; Another group of the pathogenic bacteria was similar to Vibrio fluvialis in most physiological and biochemical characteristics, but differs from the previously described V. fluvialis biovar Ⅰ and biovar Ⅱ in their grouth at 43℃, acid production from salicin, utilization of D-gluconate and putrescine, inability to utilizie glutarate and glucuronate, gas production from D-glucose and 47 mol% G+C of the base compositions. Therefore, a new biovar, Vibrio fluvialis biovar Ⅲ. biovar nov. is proposed.