A rod-shaped and Gram-positive anaerobic bacterium, named Niu-O16, which was isolated from bovine rumen contents, was found to be capable of anaerobically converting isoflavones daidzein and genistein to dihydrodaidzein (DHD) and dihydrogenistein (DHG), respectively. The metabolites DHD and DHG were identified using EI-MS and NMR spectrometric analyses. Stereoisomeric metabolites, which were separated on chiral stationary phase HPLC, were formed in equal amounts by the strain Niu-O16. Tautomerization reaction occurred on the B-ring of DHD and DHG seems to be attributed to the equal production of stereoisomeric metabolites. For the synthesis of DHD, the strain Niu-O16 showed an optimal pH range from 6.0 to 7.0 and completely reduced up to 800 microM of daidzein to DHD with the initial OD600nm=1.0 and pH 7.0 for 3 days incubation. The strain Niu-O16, showed relatively faster reduction activity toward daidzein to produce DHD than the previously isolated human intestinal bacterium Clostridium sp. HGH6.