An obligate anaerobic, Gram-staining-negative, mesophilic, cellulolytic bacterium, strain H1T, was isolated from the rumen content of yak. Cells were straight to slightly curved rods, 0.8–1.0×3.0–4.0 μm in size, non-motile and encapsulated with mucous materials. Elliptical and terminal spores that swelled the cells were produced occasionally. The strain grew at 25–45 °C (optimum, 38 °C) and pH 6.0–7.8 (optimum, pH 6.7). Cellulose, cellobiose, xylan, xylose and maltose were used as carbon and energy sources, but not glucose. Products from cellulose and cellobiose fermentation were formic acid, acetic acid, carbon dioxide and trace amounts of ethanol, lactic acid and succinic acid. The genomic DNA G+C content was 33.7±1.2 mol%. The predominant fatty acids were C16 : 0 (27.1 %), C14 : 0 (9.2 %) and iso-C16 : 0 (6.4%). Based on the 16S rRNA gene sequence analysis, strain H1T was affiliated to the clostridial rRNA cluster XIVb and showed the highest 16S rRNA gene sequence similarity to Clostridium lentocellum DSM 5427T (96.0 %). These two strains formed a distinct lineage of the family ‘Lachnospiraceae’. Based on data from this polyphasic taxonomic study, a new genus, Cellulosilyticum gen. nov., is proposed. Cellulosilyticum ruminicola sp. nov. is proposed for strain H1T. The type strain of Cellulosilyticum ruminicola sp. nov. is strain H1T (=CGMCC 1.5065T=JCM 14822T). Clostridium lentocellum was reclassified in the new genus as Cellulosilyticum lentocellum comb. nov. (type strain RHM5T=ATCC 49066T=DSM 5427T=NCIMB 11756T).