Trehalose was the main sugar accumulated in the mycelia of Rhizoctonia solani and the trehalose content was 8% and more in dry weight mycelia irrespective of the sugar sources used for incubation. The addition of validamycin A (VM-A), an inhibitor of trehalases, increased the trehalose content even more. The inhibition of growth of the mycelia of R. solani by VM-A occurred when the fungus was incubated in a medium containing trehalose as the sole carbon source but not when incubated in a medium containing various sugars and carbohydrate polymers. The results indicated that trehalose was used as a storage molecule to supply glucose required for growth of R. solani and that the anti-fungal activity of VM-A resulted from the anti-trehalase activity of the chemical.