To determine bacterial communities involved, directly or indirectly, in the anaerobic degradation of cellulose, we conducted a microcosm experiment with soil treated with 13C-cellulose, 12C-cellulose, or without cellulose with analyses of DNA-based stable isotope probing (DNA-SIP), real-time quantitative PCR, and high-throughput sequencing. Firmicutes, Actinobacteria, Verrucomicrobia, and Fibrobacteres were the dominant bacterial phyla-degrading cellulose. Generally, bacteria possessing gene-encoding enzymes involved in the degradation of cellulose and hemicellulose were stimulated. Phylotypes affiliated to Geobacter were also stimulated by cellulose, probably due to their role in electron transfer. Nitrogen-fixing bacteria were also detected, probably due to the decreased N availability during cellulose degradation. High-throughput sequencing showed the presence of bacteria not incorporating 13C and probably involved in the priming effect caused by the addition of cellulose to soil. Collectively, our findings revealed that a more diverse microbial community than expected directly and indirectly participated in anaerobic cellulose degradation.