Cellulolytic, strictly anaerobic spore-forming bacteria were isolated from chloroform treated rumen contents. They were different from previously described cellulolytic rumen clostridia in several characteristics. They formed subterminal rod-shaped spores approximately 0.7 micron by 3.5 micron. In broth cultures the growth rate was maximal at 39 degrees C and after log growth extensive autolysis occurred. Fermentation products consisted of acetate, butyrate, hydrogen and ethanol. The GC content was 31%.