Fed-batch fermentation of glucose by a mutant strain of C. thermoaceticum resulted in acetate concentrations of 83-100 gL(-1). Excess nutrients were required to maintain cell viability, especially when high cell concentrations were used. The strain was tolerant to high levels of Na, Ca and Mg. Product yield was 0.74-0.80 g acetate/g glucose, and the productivity was 0.60-0.85 gL(-1)h(-1).