Phenotypic and phylogenetic studies were performed on new isolates of a novel Gram-stain-positive, anaerobic, non-sporulating, rod-shaped bacterium isolated from a thermophilic biogas plant. The novel organisms were able to degrade crystalline cellulose. 16S rRNA gene comparative sequence analysis demonstrated that the isolates formed a hitherto unknown subline within the family Lachnospiraceae. As a representative of the whole group of isolates, strain T3/55T was further characterized. The closest relative of T3/55T among the taxa with validly published names is Mobilitalea sibirica, sharing 93.9% 16S rRNA gene sequence similarity. Strain T3/55T was catalase-negative, indole-negative, and produced acetate, ethanol and propionic acid as major end products from cellulose metabolism. The major cellular fatty acids (>1%) were 16 : 0 dimethyl acetal, 16 : 0 fatty acid methyl ester and 16 : 0 aldehyde. The DNA G+C content was 36.6 mol%. A novel genus and species, Herbinix hemicellulosilytica gen. nov., sp. nov., is proposed based on phylogenetic analysis and physiological properties of the novel isolate. Strain T3/55T ( = DSM 29228T = CECT 8801T), represents the type strain of Herbinix hemicellulosilytica gen. nov., sp. nov.