A Mo6+-reducing bacterium (strain 48), which grew on medium supplemented with 200 mM Mo6+, was isolated from stream water obtained from Chengkau, Malaysia. The chemical properties of strain 48 conform to the characteristics of Enterobacter cloacae. Under anaerobic conditions in the glucose-yeast extract medium containing phosphate ion (2.9 mM) and Mo6+ (10 mM), the bacterium reduced Mo6+ to form molybdenum blue. Approximately 27% of Mo6+ added to the medium was reduced after 28 h of cultivation. The reduction of Mo6+ with glucose as an electron donor was strongly inhibited by iodoacetic acid, sodium fluoride, and sodium cyanide, suggesting an involvement of the glycolytic pathway and electron transport in Mo6+ reduction. NADH and N,N,N',N'-tetramethyl-p-phenylenediamine served as electron donors for Mo6+ reduction. When NADH was used as an electron donor, at first cytochrome b in the cell extract was reduced, and then molybdenum blue was formed. Sodium cyanide strongly inhibited Mo6+ reduction by NADH (5 mM) but not the reduction of cytochrome b in the cell extract, suggesting that the reduced component of the electron transport system after cytochrome b serves as an electron donor for Mo6+ reduction. Both ferric and stannous ions strongly enhanced the activity of Mo6+ reduction by NADH.