Clostridium thermoaceticum ferments xylose, fructose, and glucose with acetate as the only product. In fermentations with mixtures of the sugars, xylose is first fermented, then fructose, and last, glucose. Fructose inhibits the fermentation of glucose, and this inhibition appears to be due to a repression of the synthesis of an enzyme needed for glucose utilization. Addition of metals to the culture medium increases the cell yield drastically from about 7 to 18 g per liter, and Y(glucose) values between 40 and 50 are obtained. According to the postulated pathways of the fermentation of glucose and synthesis of acetate from CO2 by C. thermoaceticum, 3 mol of ATP are available as energy for growth. Thus a Y(adenosine 5'-triphosphate) of 13 to 16 is obtained. Because the normal Y(ATP) value is 10.5, this could mean that an additional source of ATP is available by an unknown mechanism. The addition of metals also increases the nicotinamide adenine dinucleotide phosphate-dependent formate dehydrogenase activity, the overall reaction ("CO2-p acetate), and the incorporation of the