An iridovirus-like agent (WSIV) associated with fatal infections of the integument of juvenile white sturgeon Acipenser transmontanus was isolated from fish that exhibited gross and microscopic signs of the disease. The virus induced cell enlargement and slowly progressive degeneration of a recently established cell line from white sturgeon spleen (WSS-2). Virus replication occurred at 10, 15 and 20 degree C but not at 5 or 25 degree C. The most rapid growth occurred initially at 20 degree C but the greatest concentrations (10 super(6) TCID sub(50)/ml) of cell-free virus were detected at 10 and 15 degree C. Numerous virions were observed in the cytoplasm of infected WSS-2 cells and ca 70% of the infectious virus remains cell-associated. Virions possessed an external capsid with an envelope of 260 to 280 nm in diameter that surrounded an inner capsid with a dense nucleoid. Residual viral infectivity (ca 1%) was found following incubation of the virus at a temperature of 56 degree C for 30 min. The WSIV genome is presumably DNA since 50 mu g/ml 5-bromo-2'-deoxyuridine completely inhibited viral replication in the WSS-2 line.