Abstract: A process for preparing 5,6-dihydro-4-hydroxy-(S)-6-methyl-thieno[2,3-b]thiopyran-7,7-dioxide having the formula (II): ##STR1## which comprises subjecting 5,6-dihydro-4-oxo-(S)-6-methyl-thieno[2,3-b]thiopyran-7,7-dioxide having the formula (I): ##STR2## to the action of a microorganism to reduce the thienothiopyran derivative having the formula (I), and collecting 5,6-dihydro-4-hydroxy-(S)-6-methyl-thieno-[2,3-b]thiopyran-7,7-dioxide having the formula (II) from the reaction mixture, wherein said microorganism belongs to the genus selected from the group consisting of Ambrosiozyma, Arthroascus, Ashbya, Candida, Cryptococcus, Hansenula, Kluyveromyces, Lipomyces, Lodderomyces, Metschnikowia, Pachysolen, Pichia, Rhodosporidium, Rhodotorula, Saccharomyces, Saccharomycopsis, Schwanniomyces, Sporidiobolus, Spolobolomyces, Stephanoascus, Trigonopsis, Trichosporon and Wingea, according to which, a simple and effective. process for preparing 5,6-dihydro-4-hydroxy-(S)-6-methyl-thieno-[2,3-b]thiopyran-7,7-dioxide.