Three denitrifying bacteria, which were isolated from rice paddy sediment in central Taiwan, have been found capable of obtaining energy by converting nitrates to nitrogen gas under either aerobic or anaerobic conditions. The denitrification rate of the isolate DCB-T6, in particular, was not affected by the presence of oxygen, whereas the other two isolates, DCB-T23 and T25, were. A defined synthetic medium was used to confirm that these bacteria could convert nitrates to nitrogen gas under aerobic conditions. The denitrifying potential of these bacteria was evaluated when mixed with acclimated activated sludge to treat a synthetic wastewater under a fully aerobic condition. When added to a nitrifying activated sludge, the organic-nitrogen enriched T6 performed denitrification at a rate of 360 mg N/g MLVSS per day.