A virus was isolated from Siberian sturgeon Acipenser baeri and bester (beluga Huso huso × sterlet A. ruthenus hybrid) fingerlings in SSO-2, SSF-2 and WSSK-1 cell lines during an acute epizootic on a large fish farm producing fertilised sturgeon eggs and fry. Transmission electron microscopic examination of samples from both inoculated cell cultures and skin of affected fish revealed viral particles with a herpesvirus-like morphology. The etiological role of the Siberian sturgeon herpesvirus (SbSHV) was confirmed by fulfilment of Rivers' postulates. Experimental immersion of healthy Siberian sturgeon fingerlings in a suspension of SbSHV resulted in 100% mortality with signs of focal epidermal hyperplasia, skin necrosis and multiple skin haemorrhages. While infecting different organs and tissues, the virus showed clear integumentary tropism. Carp fry and rainbow trout fingerlings were neither susceptible to the virus nor did they transmit it to healthy Siberian sturgeon. KEY WORDS: Siberian sturgeon. Herpesvirus . Pathogenicity Resale or republication not permitted without written consent of the publisherDis Aquat Org 86: 193-203, 2009 In the spring of 2006, an acute disease outbreak occurred in a large warm water sturgeon hatchery situated in Tver Province in the European part of Russia. The outbreak resulted in a mass mortality of fry and fingerlings, with Siberian sturgeon Acipenser baeri being the most susceptible fish among several sturgeon species cultured there. Up to 100% mortality was registered in some fish lots.The disease developed at a water temperature range of 14 to 19°C. Affected fish were lethargic, consumed no food, had a lighter colour than normal and remained at the rearing tank bottom. Multiple haemorrhages on the ventral part of the rostrum, around the mouth and on the abdominal and lateral body surfaces were the most prominent clinical signs. In moribund fish, the liver was extremely pale, almost white. Saprolegnia sp. and myxobacteria were also found in the skin and gill tissues of many of the diseased fish. However, treatments against these secondary invaders did not result in substantial improvement of fish condition. After parasites and bacteria were excluded as possible causes of the disease, affected tissues from diseased fish were sampled for virological examination. MATERIALS AND METHODSVirus isolation. For virological examinations, samples of affected mouth and gill tissues were collected from 17 fingerling Lena sturgeons (a race of Siberian sturgeon) aged 1.5 to 4 mo old, and from 2 fingerling 3 mo old besters (great sturgeon Huso huso × sterlet Acipenser ruthenus hybrids) each with a body weight between 3 and 25 g. The collected samples were pooled from 2 to 7 fish and transported to the laboratory in chilled (2 to 4°C) condition in Eagle's minimal essential medium (MEM) supplemented with 10% foetal calf serum (FCS; Life Technologies), penicillin (300 I.U. ml -1 ) and streptomycin (300 µg ml -1 ). The time interval between sampling the fish tissues and their pr...