Brownspotted grouper (Epinephelus malabaricus), 2.5 to 15.0 cm in body length, showed a "paralytic syndrome", comprising lethargy, dark body coloration, loss of appetite, and corkscrew swimming. Diseased fish were collected from net cages in several areas in Thailand when water temperature was 28 to 30 C. Histopathological changes consisted of vacuolation in the eyes and brain. Electron microscopic examination of the eye and brain revealed the presence of numerous cytoplasmic, non-enveloped virus-like particles with hexagonal shape, about 20 nm in diameter. An experimental infection using the filtrate of homogenized diseased fish resulted in 70% mortality and reproduced the vacuolation in the eyes and brain. From these results, the disease was diagnosed as viral nervous necrosis (VNN), which has been reported in larvae and juveniles of several marine fishes.