Biological sulfate reduction was investigated at the bench and pilot scales in order to determine optimum culture conditions. Efficient strains of sulfate-reducing bacteria (SRB) were selected by classical microbiological methods and by mutagenesis. Improvement factors, including stripping, scale-up, sulfate,and organic substrate concentrations, have been studied in batch bioreactors. Two types of pilot-scale bioreactors have been adopted, the first being completely mixed with free cells and the second having two stages with immobilized cells on a fixed bed. An overall bioconversion capacity of 11 kg/m(3) . d of gypsum and 1.2 kg/m(3) . d of dissolved organic carbon has been achieved in the two-stage bioreactor.