Sporogenous hyphae and teliospores of Ustilago esculenta, collected from swollen culms of Zizania caduciflora, contained mannitol as the principal carbohydrate, together with erythritol and trehalose in trace amounts. In vitro experiments with labelled sugars showed that both sporidia and mycelium of U. esculenta incorporated fructose and glucose to a similar degree, yielding mannitol as the principal endogenous carbohydrate, erythritol and five amino acids including alanine and arginine. Sucrose was the main translocated sugar in the leaf but the principal sugars in the culms were fructose and glucose. Fructans of high mol. wt were not abundant in the culms but low mol. wt fructans (including fructose) may be important in the metabolism of the culms; the content of such fructans was highest in the non-sporulating culms. A scheme is proposed for the metabolism of carbohydrates in the formation of gau sun.