Continuous hydrogen (H-2) production during fermentation of alpha-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 H-2 production and determine Soluble end-product synthesis patterns during fermentation Of a Cellulosic Substrate under continuous culture conditions. A 5 L working volume fermentor was established and growth experiments were maintained for over 3,000 h. Substrate concentrations were varied from 1 to 4 g/L and the feed was introduced with continuous nitrogen gas sparging to prevent clogging of the feed-line. The pH and temperature of the reactor were maintained at 7.0 and 600 degrees C, respectively, throughout the Study. At concentrations above 4 g/L, the delivery of alpha-cellulose was impaired due to feed-line clogging and it became difficult to maintain a homogenous suspension. The highest total gas (H-2 Plus CO2) production rate, 56.6 mL L-1 h(-1), was observed at a dilution rate of 0.042 h(-1) and Substrate concentration of 4 g/L. Under these conditions, the H-2 production rate was 5.06 mmol h(-1). 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. Despite a number of short-term electrical and mechanical failures during the testing period, the system recovered quickly, exhibiting substantial robustness. A carbon balance was completed to ensure that all end-products were accounted for, with final results indicating near 100% carbon recovery. This study shows that long-term, stable H-2 production can be achieved during direct fermentation of an insoluble cellulosic substrate under continuous culture conditions.