The first occurrence of “viral epidermal hyperplasia" of Japanese flounder larvae was recorded in Hiroshima Prefecture in 1985, and since then the same disease has been prevailing in hatchery-reared flounder in various districts of Japan. In the present study, we applied fluorescent antibody technique (FAT) to the disease as a diagnostic procedure. Electron microscopy and transmission experiments revealed that 4 out of 5 cases examined were due to viral epidermal hyperplasia. FAT test, using a rabbit antiserum against virus particles collected from epithelial cells of affected flounder larvae, exhibited a positive reaction on only the epidermis of the fish in the above 4 epizootics. Thus, the FAT method proved to be useful for rapid diagnosis of the present disease.