The biotransformation of a sesquiterpene having a guaiane skeleton, namely ( )-α-bulnesene was investigated using the plant pathogenic fungus, Glomerella cingulata as a biocatalyst. ( )-α-Bulnesene was oxidized at the double bond of the isopropenyl group and hydroxylated at the allylic methyl group to (4 S ,5 S ,7 R )-1(10)-guaien-11,13,15-triol.