Abstract: This paper f irst reports the splenomegaly disease of seawater-cage cultured Lateolabrax japonicus. The main symptom of diseased fish is splenomegaly. Mortality rate of diseased f ish reaches 50%. Neither parasites were found on body surface, gill filament s and other organs by microscope exmination, nor pathogenic bacteria isolated from internal organs. However, under an electron microscope, a lot of virus part icles were observed in spleen and liver of the diseased f ish. The virus was a non-envelope with hexagon in section and about 180- 220nm in diameter. The virus was assigned to Iridoviridae. Cytopathological changes showed that the cells swelling and ultrastructure broke w ith large area vacuoles in cytoplasm. A lot of distinctive lymphocytes were found in blood cells, which occupies 50% of the total number of the lymphocytes.