The objective of this research was to understand how carbon loading influences hydrogen (H(2)) synthesis and metabolic flow patterns in the thermophilic, cellulolytic bacterium, Clostridium thermocellum. C. thermocellum was cultivated in batch cultures with high (5 g L(-1)) and low (1 g L(-1)) initial concentrations of alpha-cellulose at 60 degrees C. The growth rate of C. thermocellum was 22% lower (0.15 h(-1)) in cultures with low-cellulose concentration compared with cultures with high-cellulose concentrations. Although substrate depletion coincided with the end of log-growth in low-cellulose cultures, the prime reason for growth arrest in high-cellulose cultures was not identified. Ethanol, acetate, and formate were the major soluble end-products with concomitant release of H(2) and CO(2) under both conditions. Lactate appeared during the late log phase in high-carbon cultures when pH dropped below 6.4 and became the major end-product in stationary phase. During the exponential phase of cell growth, significantly higher yields for H(2) and acetate (1.90 +/- 0.14 and 1.11 +/- 0.04 mol/mol glucose equivalent, respectively) were obtained from low-cellulose cultures compared to those from high-cellulose cultures. The maximum specific rate of H(2) production, 6.41 +/- 0.13 mmol H(2)/g dry cell/h, obtained during the exponential phase from low-carbon cultures was about 37% higher than that obtained from high-carbon cultures.