The degradation of cellulose by Clostridium cellulolyticum has been studied in several ways: (1) in batch fermentation in 50-ml sealed-cap flasks, referred to as the control; (2) in batch fermentation with pH at 7.2; (3) fermentation in dialysis which permits elimination of all the products of metabolism; (4) fermentation in dialysis with a constant bubbling of nitrogen; (5) in co-culture with Clostridium A22 in batch with and without pH regulation and with dialysis. H2, CO2, acetate, ethanol and lactate were the major end-products of cellobiose and cellulose fermentation. Compared to batch culture, growth of Cl. cellulolyticum on cellobiose increased by a factor of 10 in dialysed culture. The end products from the dialysed culture were detected in a small range compared to the concentration for the batch culture. Related to the biomass, CMCase activities were of the same level, showing a direct relation between the biomass formation and the cellulase production. The percentage of cellulose degradation (50%) by Cl. cellulolyticum was greater when dialysis of end products with a constant bubbling of nitrogen took place during the course of fermentation (6 d) in comparison with cultures in 50-ml sealed-cap flasks (23%), in a fermentor (36%) or using dialysis without N2 bubbling (40%). The presence of two micro-organisms produced no further enzyme activities and hence the percentage of cellulose degradation was quite similar in mono- and co-culture. No synergistic action was found between two cellulolytic strains.