Kinetics of [1-13C]glucose utilization were monitored by in vivo NMR spectroscopy on resting cells of Fibrobacter succinogenes, in the presence of 32 mM [1-13C]glucose, 32 mM [1-13C]glucose and 64 mM unlabelled glucose, or 32 mM [1-13C]glucose and 32 mM unlabelled cellobiose. A similar production of acetate and succinate and a similar storage of glycogen were observed whatever the exogenous substrate. The presence of cellobiose or that of an equivalent amount of glucose did not reduce [1-13C]glucose incorporation to the same extent. Glucose seemed preferentially used for glycogen storage and energy production, while part of the cellobiose appeared to be used for cellodextrin synthesis. Both cellobiase and cellobiose phosphorylase activities were assayed in cell-free extracts. Finally, the intracellular concentration of glucose 6-phosphate was increased by over threefold when cells metabolized cellobiose (alone or in parallel to glucose) as compared with the metabolism of glucose alone.