The conventional chemical reduction of Cr(VI) to Cr(III) and subsequent Cr(OH), precipitation are expensive due to the use of large amounts of chemicals and the generation of chemical sludges. An attempt was carried out for microbial Cr(VI) removal in an anaerobic chemostat fed with an acetate-containing synthetic medium. With 26 mg Cr(VI) dm(-3) in the influent, almost complete removal of Cr(VI) was achieved at dilution rates of 0.15 and 0.32 day(-1) at 20 degrees C and at 35 degrees C, respectively. The optimum Cr(VI) mass loading and the specific Cr(VI) applied rates were found to be 5 mg Cr(VI) dm(-3) day(-1) and 0.02 mg Cr(VI) mg(-1) VSS day(-1), respectively. Either the influent Cr(VI) concentration or the dilution rate could be adjusted to maintain an efficient removal of Cr(VI) in a continuous operation. Since the Cr(VI)-reducing activity is associated with the biomass concentration in the system, recycling the effluent solids is essential for practical application. In a batch reactor with the biomass collected from the chemostat, NaAc degradation appeared to be proportional to Cr(VI) reduction with the ratio of 9 mg C mg(-1) Cr(VI) at 35 degrees C. As reactions proceeded, the oxidation-reduction potential correspondingly decreased and both pH and alkalinity increased.