We have isolated a large virus from pike-perch Stizostedion lucioperca fingerlings with no signs of disease. The biochemical structural, and serological properties of this newly isolated virus suggest that it belongs to the family Iridoviridae. The virus multiplied and was cytopathogenic in several cultured fish cell lines. The virus has a DNA-containing genome and is assembled in the cytoplasm. When viewed in electron micrographs, the assembly sites showed a paracrystalline array of hexagonal nucleocapsids. The ultrastructure of the pike-perch virus resembled that of previously isolated fish iridoviruses. It is an enveloped icosahedral DNA virus. The diameter of the nucleocapsid in thin sections was 127 +/- 3 nm; in negatively stained preparates the size of the enveloped virus varied from 147 to 187 nm. In immunofluorescence the virus was stained by rabbit antisera against EHN (epizootic haematopoietic necrosis) virus, sheatfish iridovirus and cod iridovirus. The pathogenicity of the virus isolate was studied by inoculation into juvenile rainbow trout Oncorhyncus mykiss. Experimental infection under aquarium conditions suggested that the virus is apothogenic to rainbow trout. The infective virus could be recovered from the viscera of inoculated fish during the first week post-infection, after which the proportion of virus-positive fish declined over time. A small proportion of the fish still carried the virus 24 d post-inoculation.