The objective of this research was to understand how carbon loading influences hydrogen (H2) synthesis and metabolic fluxes in a thermophilic, cellulolytic bacterium, Clostridium thermocellum. C. thermocellum was cultivated in batch cultures with high (5 g/liter) and low (1g/liter) initial concentrations of alpha-cellulose at 60°C. The growth rate (as determined by protein concentration) of C. thermocellum was 17% lower (10.62 h) in cultures with low cellulose concentration compared with cultures with high-cellulose concentrations. At stationary phase, cell mass was higher (63.3 milligrams/liter) in high-cellulose cultures compared with low-cellulose cultures (42.8 milligrams/liter). Although substrate depletion coincided with the end of log-growth in low-cellulose cultures, a drop in pH below 6.0 is suspected to be the prime reason for growth arrest in high-cellulose cultures. Ethanol, acetate, and formate were the major soluble end-products, with concomitant release of H2 and CO2 under both conditions. Lactate appeared during late-log phase in high-carbon cultures when pH dropped below 6.4 and became the major end-product in stationary phase. Higher yields for H2 and acetate (1.73 moles and 1.16 moles / mole glucose) and lower yields for ethanol (0.49 moles / mole glucose) were obtained from low-cellulose cultures compared to those from high-cellulose cultures (1.53 moles H2, 0.82 moles acetate and 0.68 moles ethanol per mole glucose). The maximum specific rate of H2 production, 5.88 millimoles of H2/g dry cell/h, obtained from log phase of low-carbon cultures was about 37% higher than that obtained from high-carbon cultures.