Epidemiol. description, pathol. and etiol. 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.Totally 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 hemorrhagic in liver and kidney, serious hemorrhagic and edema in alimentary canal, hemorrhagic and necrosis in gill-like tissue.The major histol. 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 epidemiol. investigation and pathol. 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.Morphol. observation revealed that the strain of bacterium was a middle-sized Brevi-bacterium without capsule and brood cell arrayed by single or two strains, two ends of which were obtuse circle.Hemolysis anal. showed that the strain of bacterium grew well and presented obvious β hemolysis on the blood agar plate.Measurement of protein enzyme revealed pos.Physiol. and biochem. tests indicated that the isolated bacterium decomposed glucose, arabinose, sucrose, maltose, esculoside, mannitol and salicin; it did not ferment xylose, lactose and sorbitol; tests of M R, V P and oxidase were all pos.16S rRNA sequence anal. 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), resp.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 Acipenser baerii cultivation.