Perchlorate is a man-made and naturally occurring anion that causes hypothyroidism and is often present in groundwater together with nitrate. In this study, a novel elemental sulfur-based denitrifying membrane bioreactor was tested for simultaneous nitrate and perchlorate reduction. A bench scale 4 L bioreactor equipped with polyethersulfone (PES) ultrafiltration membranes (0.04 mu m of pore size) was used. Influent nitrate was varied between 25 and 100 mg NO3--N/L. Complete denitrification of 100 mg NO3--N/L was achieved at HRT as low as 6 h corresponding to 400 mg NO3--N/(L.d) denitrification rate. Complete reduction of perchlorate was also achieved when the influent concentration was 3000 mu g/L and HRT of 12 h. Further increase in perchlorate load resulted in elevated effluent perchlorate concentration and maximum perchlorate reduction rate was 15.1 mg/(L.d). Sulfate was produced as a result of sulfur based denitrification. Influent was supplemented with NaHCO3 (1000 mg CaCO3/L) to meet the alkalinity requirement of the process. Membrane fouling was not significant at 12.5 L/(m(2).h) membrane flux.