The effect of light and culture media associated with buoyancy of sclerotia of Corticium rolfsii,causal agent of southern blight disease of water chestnut (Trapa bispinosa Roxb.) was examined. Sclerotia formed in field became buoyant in water, but sclerotia on culture media sunk in water. Microscopic study revealed that sclerotia formed in field had more intercelluler spaces in sclerotial tissues and thinner cell walls than sclerotia on culture media. Many sclerotia were formed under light conditions, although buoyancy of sclerotia depended on both the light conditions and the nutrient constituents of the medium. When diluted potato sucrose agar (PSA) or water chestnut leaves agar media were used for formation of sclerotia under light conditions, buoyant ability and internal structures were similar to that of sclerotia formed in tield. On the other hand, fewer sclerotia were formed on each kind of media under dark conditions, but the sclerotia did not float in water. Light conditions and nutrient constituents play important roles for the formation of buoyant sclerotia of C. rolfsii.