A mesophilic anaerobe, a member of the Bacteroidaceae family (NRC2248), isolated from a cellulose-enrichment culture, digested untreated Whatman cellulose powder and HCl-treated cotton battings while producing hydrogen, carbon dioxide, cellobiose, glucose, and acetic acid as the sole volatile acid. This organism also utilized cellobiose as carbon and energy source but did not utilize glucose. It grew well in synthetic medium containing ammonium salts as nitrogen source and having a pH value of 7.0-7.1 and an Eh value of -160mV or lower. The nutrient requirements of this organism were found to be similar to those of other anaerobes except for Na2S which inhibited cellulose degradation in concentrations above 0.75 mM. Best cellulose degradation occurred under an atmosphere of 80% N2-20% CO2. Use of H2 or 80% H2-20% CO2 as headspace gas inhibited growth. Although accumulation of acetic acid in about 15-16 mM concentrations inhibited the further formation of H2, CO2, and acetic acid in the broth, it did not stop the degradation of cellulose. The results indicate that this organism has the ability to grow in media containing up to 20 g/L of cellulose and to produce industrially important and easily separable end products from cellulose.