An automated repetitive batch fermentation system was developed to facilitate the study of microbial cellulose utilization. The system was operated with Avicel as the carbon source and either Clostridium thermocellum ATCC 27405 or a consortium enriched from compost as inocula. Multiple cycles of growth medium addition, incubation, and medium removal were performed with each inoculum. Removal and addition of media were automatically initiated when CO2 production fell to 90% of the cycle's peak. A strong correlation was observed between CO2 production and cellulose consumption, suggesting that the online signal was a good proxy for substrate utilization. Both cultures exhibited accelerated substrate utilization and a decrease in cycle time. About the same number of cycles was required to reach maximum CO2 production for both cultures. Notably, the magnitudes of the maximum CO2 production rate and cycle times were very similar for both C. thermocellum in pure culture and an environmental consortium. (C) 2013 Elsevier Ltd. All rights reserved.