( )-Menthofuran is an undesirable monoterpenoid component of peppermint (Mentha x piperita) essential oil that is derived from the ,-unsaturated ketone ( )-pulegone. Microsomal preparations, from the oil gland secretory cells of a high ( )-menthofuran-producing chemotype of Mentha pulegium, transform ( )-pulegone to ( )-menthofuran in the presence of NADPH and molecular oxygen, implying that menthofuran is synthesized by a mechanism analogous to that of mammalian liver cytochrome P450s involving the hydroxylation of the syn-methyl group of ( )-pulegone, spontaneous intramolecular cyclization to the hemiketal, and dehydration to the furan. An abundant cytochrome P450 clone from a peppermint oil gland cell cDNA library was functionally expressed in Saccharomyces cerevisiae and Escherichia coli and shown to encode the ( )-menthofuran synthase (i.e., ( )-pulegone-9-hydroxylase). The full-length cDNA contains 1479 nucleotides, and encodes a protein of 493 amino acid residues of molecular weight 55,360, which bears all of the anticipated primary structural elements of a cytochrome P450 and most closely resembles (35% identity) a cytochrome P450 monoterpene hydroxylase, ( )-limonene-3-hydroxylase, from the same source. The availability of this gene permits transgenic manipulation of peppermint to improve the quality of the derived essential oil.