Clostridium thermocellum produces ethanol, acetate, H(2), and CO(2) as major fermentation products from cellulose and cellobiose. The performance of three strains of this microorganism was studied to assess the potential use in producing ethanol directly from cellulosic fiber. Depending on the bacterial strain, an ethanol/acetate product ratio from 1 to as high as 3 was observed in unstirred cultures. Vigorous stirring during growth resulted in a threefold decrease in the ethanol/acetate ratio. The H(2) content in the unstirred culture broth was three times greater than that in the stirred one. Addition of exogenous H(2) to the gas phase during growth increased the ethanol/acetate ratio much more in the stirred than in the unstirred fermentations. The addition of sufficient H(2) to the gas phase almost relieved the effect of stirring, and the ethanol/acetate ratio approached that in the unstirred condition. Addition of tritium to the gas phase of the culture resulted in the formation of tritiated water (H(2)O), which indicates that C. thermocellum possesses hydrogenase(s) that catalyzes the reverse reaction. The rate of H(2)O formation was about three times higher in the stirred culture than in the unstirred culture. These results demonstrate that the H(2) concentration in the broth plays an important role in the product formation. The H(2) supersaturation present in the unstirred cultures is responsible for the observed effect of stirring. A hydrogen feedback control mechanism regulating the relative concentrations of reduced and oxidized electron carriers is proposed to account for the effect of hydrogen on the metabolite distribution.