Abstract Dihydrooxoisophorone (DOIP, 2,2,6-trimethyl-1,4-cyclohexanedione) was converted to 4-hydroxy-2,2,6-trimethylcyclohexanone (4-HTMCH) by a thermophile isolated from soil. The thermophile was identified as Bacillus stearothermophilus. 4-HTMCH has four isomers: cis-(4 R, 6 S)-, cis-(4 S, 6 R)-, trans-(4 R, 6 R)- and trans-(4 S, 6 S)-. Their quantitative ratio was 68:25:5:2. The extent of DOIP conversion was 75% in batch culture after 20 h cultivation and 73% in continuous culture at a dilution rate of 0.07 h −1 in an air-lift reactor. The maximum production rate of 4-HTMCH in continuous culture in an air-lift reactor was 200 mg l −1 h −1 at a dilution rate of 0.12 h −1. This microbial conversion was closely associated with cell growth. The relationship between conversion of DOIP and cell growth was also investigated.