Abstract: Epidemiological description,pathology and etiology of hemorrhagic septicemia in infected Siberian sturgeon(Acipenser baerii) were studied in this paper.The outbreak of this infectious and lethal disease happened in a fish farm which was located in Linfen,Shanxi Province,China.20% of cultured Acipenser baerii was infected and the mortality rate was 15%.Fish suffering from hemorrhagic septicemia had many symptoms and these ranged from emaciation and red sore mouth to skin ulcerations at any site of the fish.Postmortem examination revealed swelling and haemorrhagic in liver and kidney,serious haemorrhagic and edema in alimentary canal,haemorrhagic and necrosis in gill-like tissue.The major histological lesions were epithelium degeneration,necrosis and desquamation in intestine mucous membrane;degeneration,necrosis in gastric wall smooth muscle;granular and vesicular degeneration and necrosis in renal tubular epithelial cells.On the basis of epidemiological investigation and pathological observation,the bacterium was isolated from the focal area of the infected Acipenser baerii,and it was dominant among the isolates.The symptom of the artificial infected fish and the natural infected fish was similar.Morphological observation revealed that the strain of bacterium was a middle-sized Brevibacterium without capsule and brood cell arrayed by single or two strains,two ends of which were obtuse circle.Hemolysis analysis showed that the strain of bacterium grew well and presented obvious β hemolysis on the blood agar plate.Measurement of protein enzyme revealed positive.Physiological and biochemical tests indicated that the isolated bacterium decomposed glucose,arabinose,sucrose,maltose,esculosied,mannitol and salicin;it did not ferment xylose,lactose and sorbitol;tests of M R,V P and oxidase were all positive.16S rRNA sequence analysis showed the sequence length was 686 bp.The nucleotide sequence of isolated bacterium was 100%,99% and 98% identical to that of Aeromonas hydrophila published by Hebei Teacher-training University(gi:119633076) and Sichuan Agricultural University(gi:188013261),Japan(gi:188496906) and India(gi:188032688),CAS(gi:189031545),respectively.Thus,the strain of bacterium was identified as Aeromonas hydrophila according to these methods.The drug sensitivity test showed that it was highly sensitive to CTX,KAN,GEN,TET,CMP,POL,NOR,CIP,ENR and resistant to AMO.This paper will be helpful to the disease control and health management during Asipenser baerii cultivation.