This report describes the use of a new selenate-respiring bacterium, Thauera selenatis, for the bioremediation of selenium (Se, as selenate) in drainage water from the Westlands Water District, San Joaquin Valley, The organism respires selenate anaerobically using acetate as the preferred electron donor. The reduction of selenate is not inhibited by nitrate; both electron accepters are reduced concomitantly. T. selenatis was inoculated into, and was maintained in, a biological reactor system for anaerobic treatment of selenate-nitrate containing drainage water; a population of denitrifying bacteria was also present. When the pH of inflowing water was 6.9, and 2 mM acetate plus 0.56 mM ammonium chloride were fed into the reactor, selenate/selenite levels were reduced from 350-450 mu g Se/l to 5.39 +/- 3.6 mu g Se/l. The final product of selenate reduction was elemental Se. Analysis of reactor contents revealed that T. selenatis was the only selenate-respiring organism present in the system. Nitrate in the drainage water was also reduced in the reactor system by 98%. The lab-scale biological reactor system consisted of recycled sludge-blanket (1 l; 400 g sand) and fluidized-bed (1 l; 300 g sand) reactors. At a system flow rate of 6.5 ml/min, the retention time was 140 min.