NEW MATERIAL:A validamycin A derivative of formula I [R is H, formyl or (substituted) carbamoyl; R' is H, acyl, (substituted) hydrocarbon residue, etc., when all R' are H or acetyl at the same time, R is formyl or (substituted) carbamoyl].

EXAMPLE: N-Formyl-undeca-O-acetylvalidamycin A.

USE: An agricultural and horticultural fungicide which exhibits high controlling activity against damping-off of rice plant for a long time even in non-sterilized soil, and has long-acting effect against sheath blight of rice plant.

PROCESS: For example, the acyl derivative of validamycin A can be obtained by reacting validamycin A with a carboxylic acid halide or anhydride in the presence of an inorganic or organic base such as sodium hydroxide, pyridine, etc., and if necessary, concentrated sulfuric acid, and zinc chloride, thereby esterifying the hydroxy group of the starting compound.