The biotransformation of l-menthol was investigated by using nine isolates of Rhizoctonia solani (AG-1-IA Rs24, Joichi-2, RRG97-1; AG-1-IB TR22, R147,110.4; AG-1-IC F-1, F-4 and P-1) as a biocatalyst. In the cases of Rhizoctonia solani F-1, F-4 and P-1, almost all of the substrate was consumed in 3 days and the major metabolite increased rapidly for the first of 3 days incubation. The structure of the major metabolite was elucidated on the basis of its spectral data. The major metabolite was determined to be (-)-(1S,3R,4S,6S)-6-hydroxymenthol which indicated that l-menthol was hydroxylated at the C-6 position. From the main component analysis, the nine isolates of Rhizoctonia solani were divided into two groups based on their ability to transform l-menthol to (-)-(1S,3R,4S,6S)-6-hydroxymenthol. This is the first report on the biotransformation of l-menthol by Rhizoctonia solani.