So far carbon metabolism of Clostridium cellulolyticum, a mesophilic cellulolytic anaerobic bacteria, has been only investigated with cellobiose. In batch with cellulose, this bacterium appears sensitive to the acidification while in pH-control culture the growth arrest results from a high entering carbon flow leading to the accumulation of intracellular inhibitory compound(s). The metabolic flux analysis in chemostat fed with cellulose under limited or saturated conditions of carbon and/or ammonium showed (i) the carbon entry was regulated by the cellulosome (ii) the longer the cellodextrin incorporated into the cell is, the more the glucose 1-phosphate is generated (iii) the phosphoglucomutase control the orientation of the flux towards glycolyse (iv) the glycogen buffer the carbon surplus (v) the pyruvate metabolic node is of key importance in electronic and energetic fluxes regulation (vi) the metabolic fluxes remain always lower with cellulose than those obtained with cellobiose.