A histopathological ovservation was made on a spontaneous lymphocystis disease which occurred in Lateolabrax japonicus (CUVIER and VALENCIENNES)in a fish farm pond at Kanzanji in Lake Hamana. The disease was first noticed in the early part of June, 1971 and continued till early September, 1971, when it came to an end. Part of affected fishes carried only on fins and the caudal trunk several small white warts which consisted of many lymphocystis cells. In serious cases, however, large masses of warts were found not only on the fins but also on various parts of the body surface, even on the eyes. From histological observation it was thought that lymphocystis cells in this species were originated from connective tissue cells of the skin. The lymphocystis cells had cytological features common to those described by WEISSENBERG and many other authors;that is, very large sizes, frequently over 400 μ in diamater, thick hyaline cellmembrane, large nucleus enriched with chromatic substance, large deeply basophilic nucloli, and deeply basophilic network-shaped inclusin bodies.