A novel Gram-stain-positive, rod-shaped, anaerobic, thermophilic bacterium, strain GGR1T, was isolated from a thermophilic lab-scale biogas fermenter. The novel organism was effectively degrading crystalline cellulose. It seems to play a role in remineralization of plant biomass by hydrolysing its polysaccharides. 16S rRNA gene comparative sequence analysis demonstrated that the isolate formed a hitherto unknown subline within the family Ruminococcaceae. The closest phylogenetic relative of GGR1T among the taxa with validly published names was Clostridiumthermocellum, sharing 94.3 % 16S rRNA gene sequence similarity. Strain GGR1T was catalase-negative, indole-negative and produced acetate and ethanol as major end-products during fermentative cellulose utilization. The major cellular fatty acids (>1 %) were 16 : 0 iso fatty acid and 16 : 0 fatty acid. Cells were rod shaped and grew optimally at 60 °C and pH 7.0. The DNA G+C content was 34.9 mol%. A novel genus and species, Herbivoraxsaccincola gen. nov., sp. nov., is proposed on the basis of phylogenetic analysis and physiological properties of the novel isolate. Strain GGR1T (=DSM 101079T=CECT 9155T) represents the type strain for the novel genus and novel species Herbivoraxsaccincola gen. nov., sp. nov.