A new epizootic viral disease has threatened cultured grouper (Epinephelus sp.) in Taiwan since 1995.Icosahedral viral particles with a diameter of 230 ± 10 nm were observed in the spleen of moribund fish. Cell rounding, a cytopathic effect (CPE), was induced in KRE cell line by the virus isolated from diseased groupers. Virus infectivities declined rapidly during serial passages in the KRE cell line. The susceptibility of the virus to chloroform and ether treatments indicates that the virus may be enveloped. When stained with acridine orange, the inclusions in the KRE cells exhibited a greenish fluorescence, and this together with the results of IUDR treatment suggest that the viral genome of the virus is double-stranded DNA. Based on its general properties and viral morphology, the name Grouper Iridovirus of Taiwan (TGIV) was proposed. When healthy juvenile groupers were experimentally infected by intraperitoneal injection with TGIV, cumulative mortalities reached 100% within 11 days, while no grouper died in the control groups. Virus similar in morphology to that observed in spontaneously diseased fish was then re-isolated from the experimentally infected groupers. These results suggest that this virus may be the causative agent of the epizootic disease that now afflicts cultured grouper in Taiwan.