In 1974, from June to July, a disease which was accompanied by a considerably high mortality occurred among juvenile colorcarp, Cyprinus carpio, at many farms in Niigata Pref., Japan. The outbreak of the disease has been observed in the same season every year since then. In most cases, a large number of fish died within a few days after onset of the disease. The characteristic symptom was swelling due to edema of the fish body. The purpose of the present study is to obtain the histopathological findings of the disease by light and electron microscopic observation. It was progressive change which induced club-shape transformation of gill lamellae by proliferation and thickening of epithelial cells of the secondary gill lamellae. The club-shape transformation began at the tip of the gill lamellae, gradually extends downward to spread all over the gill lamellae. As the degeneration proceeded, gill lamella-adhesions also began to occur. At this stage, the capillary cavities in secondary gill lamellae were almost occluded. In the gill filaments with marked pathological change electron microscopic observation revealed many virus-like particles in the epithelial cells of thickened secondary gill lamellae of diseased fish. The particle was about 250-280 nm in diameter and the profile was mulberry-like with an envelope. These morphological features suggest that this virus may be a member of the poxvirus group.