A new mesophilic, cellulolytic Bacteroides sp. was isolated from a methanogenic cellulose enrichment culture of municipal sewage sludge and is described. This species ferments only cellulose and cellobiose. The fermentation products are acetic acid, CO2, H2, ethanol and small amounts of lactic acid. The DNA base composition is 43 mol% guanine plus cytosine. The name B. cellulosolvens is proposed. Type strain WM2 is deposited in the National Research Council of Canada culture collection as strain NRCC 2944.