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A new biological process for selenium removal from metal refinery wastewater was developed using selenate-and selenite-reducing bacteria. Bacterial reduction of selenium oxyanions into elemental selenium plays a role in soluble selenium detoxification. First, two pilot-scale anaerobic bioreactors (256 l each) were constructed. One was inoculated with granular sludge of an upflow anaerobic sludge blanket reactor (Reactor A). The other was equipped with the acrylic biomass carrier and was inoculated with suspended sludge of a sewage sludge digestion reactor (Reactor B). However, those bioreactors inoculated with anaerobic sludge failed to remove selenium. As a bioaugmentation strategy, an aerobic selenium-reducing bacterium, Pseudomonas stutzeri NT-I, was inoculated to the bioreactors with slight aeration. Regarding selenite-containing wastewater, Reactor A, where granular sludge and P. stutzeri NT-I coexisted, removed 95 % of 65 mg-Se l -1 quickly within only 2 days. Reactor B equipped with the biomass carrier and inoculated with P. stutzeri NT-I needed 5 days for 98% selenite removal. For selenate-containing wastewater, Reactor A removed only 54% of 30 mg-Se l -1 in 7 days. In stark contrast, Reactor B showed selenium removal from selenate-containing wastewater of over 90% within only 3 days. The effluent of each reactor turned deep red, indicating formation of elemental selenium, which can be removed easily from the aqueous phase because of its insoluble characteristics.