A liquid-solid fluidized bed reactor was used to carry out sulfate reduction with a mixed cu Itu re of su if ate reducing bacteria. The bacteria were immobilized on po rous glass beads. Stable fluidized bed operation with these biofilm-coated beads was possible. The low specific gravity of the hydrated beads allowed operation at low liquid recirculation rates. H2S level in the reactor was controlled by N-2 sparging, which also served as the location for liquid feed and removal. Ethanol was used as the electron donor/carbon source for the bacteria. Sulfate reduction rates up to 6.33 g sulfate L-1 day(-1) were attained in the reactor at a hydraulic retention time of 5.1 h. The effect of hydraulic retention time and biomass loading on the beads, on reactor performance, and efficiency were examined. The efficiency of sulfate reduction increases considerably as the hydraulic retention increases, until the bacteria became very strongly substrate-limited at 55h HRT. The effect of bead biomass loading on bed expansion at various liquid superficial velocities was studied. A model for the reactor was developed. Simulations of the continuous flow experiments indicate that the model can describe the system well, and thus could be used in the design/scale-up of such reactors. The model suggests that a significant increase in the sulfate reduction capacity of the system is possible by increasing the volume of the bed relative to the total liquid volume of the system.