Two strains of obligately anaerobic, mesophilic, cellulolytic, N2-fixing, spore-forming bacteria were isolated from soil samples collected at two different locations near Amherst, MA, USA. Single cells of both strains were slightly curved rods that measured between 2 and 6 microm in length and approximately 0.5 microm in diameter. The spores were spherical, terminally located, distended the sporangium and measured 0.8-1.0 microm in diameter. The cells of both isolates (designated strain ADT and strain B3B) stained Gram-negative, but did not have a typical Gram-negative cell wall structure as demonstrated by transmission electron microscope analysis. The cells of both strains were motile with subpolarly inserted flagella and exhibited chemotactic behaviour towards cellobiose and D-glucose. Both strains fermented cellulose, xylan, cellobiose, cellodextrins, D-glucose, D-xylose, D-fructose, D-mannose and gentiobiose. In addition, strain B3B fermented L-arabinose. For both strains, fermentation products from cellulose were acetate, ethanol, H2 and CO2, as well as small amounts of lactate and formate. The G+C content of strain AD was 40 mol% and that of strain B3B was 42 mol%. Based on their morphological, physiological and phylogenetic characteristics, it was concluded that the two isolates are representatives of a novel species of Clostridium. The name Clostridium hungatei is proposed for the new species. The type strain of Clostridium hungatei sp. nov. is strain ADT (= ATCC 700212T).