Mass mortalities of sevenband grouper Epinephelus septemfasciatus reared in Oita Prefecture were investigated. The outbreaks of the disease occurred in summer seasons (July to October; water temperature 25-28 degree C) of 1993 and 1994. Affected fish ranging from 170 to 1,850g in body weight were characterized by upside down swimming and inflation of the swimbladder. Histopathologically, necrosis and vacuolation in the central nervous tissues and retina were observed. Although bacteria or parasites associated with the disease were not detected from affected fish and no CPE was observed on RTG-2, BF-2, and EPC cell lines, spherical virus particles, 28 nm in diameter, were found abundantly in the cytoplasm of affected nerve cells of most of the affected fish. The virus was identified as a fish nodavirus (nervous necrosis virus), based on the results of a fluorescent antibody technique test with anti-SJNNV rabbit serum and polymerase chain reaction test with primers for amplification of SJNNV coast protein gene. These results suggest that the mortalities of sevenband grouper were at least partly due to viral nervous necrosis (VNN). This is the first report describing VNN of sevenband grouper and the first case of VNN in the adult fish.