Pathological changes of rice sheath tissues caused by Rhizoctonia solani treated with validamycin A (VM-A) was anatomically observed. In sheaths inoculated with mycelial disks after dipping in solutions of 0.63, 1.25, and 2.5ppm VM-A, browning of sheaths and small elliptical spots arose, but no hyphae were observed in the inner space of the sheath. On the other hand, in the lessions formed by inoculation of untreated mycelial disks, abundant hyphae were present in the inner space of the sheaths and the parenchyma cells were severely damaged. Spraying of 10ppm VM-A at 8hr after inoculation of R. solani inhibited the penetration of hyphae into sheath tissues and the browning of sheaths on inoculated part or small elliptical spots were seen on the 4th day. When VM-A was sprayed 16hr after inoculation, hyphae became to be seen in the sheath tissues on the 4th day. Their growth, however, was strikingly inhibited accompanying with little damage of the tissues compared with the lesions formed by the untreated hyphae. The results demonstrate that VM-A is effective against R. solani by both preventing hyphal penetration and retarding its growth.