A novel anaerobic, mesophilic, hydrogen-producing bacterium, designated strain M2/40(T), was isolated from a mesophilic, two-phase, laboratory-scale biogas reactor fed continuously with maize silage supplemented with 5% wheat straw. 16S rRNA gene sequence comparison revealed an affiliation to the genus Clostridium sensu stricto (cluster I of the clostridia), with Clostridium cellulovorans as the closest characterized species, showing 93.8% sequence similarity to the type strain. Cells of strain M2/40(T) were rods to elongated filamentous rods that showed variable Gram staining. Optimal growth occurred at 35 °C and at pH 7. Grown on glucose, the main fermentation products were H2, CO2, formate, lactate and propionate. The DNA G+C content was 29.6 mol%. The major fatty acids (>10 %) were C(16 : 0), summed feature 10 (C(18 : 1)ω11c/ω9t/ω6t and/or unknown ECL 17.834) and C(18 : 1)ω11c dimethylacetal. Based on phenotypic, chemotaxonomic and phylogenetic differences, strain M2/40(T) represents a novel species within the genus Clostridium, for which we propose the name Clostridium bornimense sp. nov. The type strain is M2/40(T) ( = DSM 25664(T) = CECT 8097(T)).