An etiological study was conducted to identify the causative agent of contagious edema of cultured juvenile colorcarp, Cyprinus carpio, in Japan. The disease outbreaks were often observed during the rainy season, extending from late June to late July, and the mortality frequently reached 80-100%. High mortality with edema was demonstrated among carp fry (0.10-0.26g) exposed to the filtered homogenates of diseased fish. Repeated infection trials demonstrated that the disease was transmissible by using filtered homogenates even after 12 times passage.Pox-like virus particles, 335 (300-400) nm in length and 265 (245-290) nm in width, were detected in the gilllamellar epithelial cells from both spontaneously affected and experimentally infected fish. Pancreas, kidney, spleen, and intestine of experimentally infected fish showed histopathological changes, but pox-like virus or other pathogenic organisms were not found in these organs. In the case of experimentally infected fish, more than 80%of infectivity was found in gills when the titers were compared by quantitative infection trials. Although the virus has not been successfully isolated, the facts mentioned above indicate that the pox-like virus observed in gill epithelial cells is the pathogen of this disease. We suggest that this infectious disease be termed “viral edema of carp”.