A mutant strain of Clostridium thermoaceticum was studied in a cell-recycle membrane bioreactor for production of acetate from glucose. With 20-30 g l(-1) glucose in the feed and a cell concentration of 5 g l(-1), dilution rates up to 0.25 h(-1) resulted in complete utilization of substrate, resulting in a productivity of 4 g l(-1)h(-1) and acetate concentration of 16 g l(-1) in the broth. This productivity was 10-fold higher than that of the batch fermenter. Higher cell concentrations allowed higher glucose concentrations to be used, resulting in acetate concentrations of 44 g l(-1) (52.5 g l(-1) Na acetate) in the fermentation broth, although the dilution rate had to be lowered to 0.01 h(-1). High dilution rates resulted in lactate accumulation and a decrease in yield of acetate. The membrane bioreactor was run continuously under varying operating conditions for 54 days without any significant problems.