Continuous hydrogen (H2) production during fermentation of α-cellulose was established using the thermophillic, anaerobic, bacterium Clostridium thermocellum ATCC 27405. The objectives of this work were to characterize growth of C. thermocellum, quantify H2 production, and determine soluble end-product synthesis patterns during fermentation of powdered α-cellulose under continuous culture conditions. A 5 L fermentor was established and growth was maintained for over 3000 hours. During this time, the cellulose was introduced continuously with continuous nitrogen gas sparging to prevent clogging of the feed-line. The concentration of cellulose was increased stepwise from 1 to 4 g per L. The pH (7) and temperature (60°C) of the reactor were maintained throughout the experiment. At concentrations above 4 g per L, the delivery of α-cellulose was impaired due to feed-line clogging and it became difficult to maintain a homogenous suspension. The highest total gas (H2 plus CO2) production rate, 56.6 ml per L per hr, was observed at a dilution rate of 0.042 L per hour and substrate concentration of 4 g per L. Under these conditions, the H2 production rate was 5.06 mmol per hr. Acetate and ethanol were the major soluble end-products, while lactate and formate were greatly reduced compared to production in batch cultures. Concentrations of all metabolites increased with increasing substrate concentration, with the exception of lactate. These results show that H2 production is proportional to substrate concentration, but product ratios remain constant within the loading rates tested.