A new disease with high mortalities has been occurring in summer scince 1998 in different ages of carp Cyprinus carpio cultured in Korea. The clinical signs of the disease included erratic swimming of moribund fish at the water surface, respiratory distress, presence of thick and foggy mucus covering the body and gills. Dermal ulcers were also noticed in a few cases. Microbiological, parasitological and virological investigations were carried out on the fish samples collected from various localities. Although bacteria and a protozoan parasite were recovered from some fish, their occurrences were not consistent. Histologically, the kidney and spleen showed discernible changes. Inoculation of tissue filtrates from affected fish to various fish cell lines showed cytopathic effects only in FHM. Ultra-thin sections of the infected FHM cells under an electron microscope revealed distinct cytoplasmic virus-like particles with a diameter of 70-80 nm. Immersion infection trials, using a viral suspension of 10^3.0TCID₅₀/mL obtained from the infected FHM cells, produced 100 and 80% mortalities in Israel carp and common carp, respectively. This is the first epizootic of viral disease reported among cultured carp in Korea.