An anaerobic, mesophilic, spore-forming, cellulolytic bacterium was repeatedly isolated from a wood-fermenting anaerobic digester. Cells of this organism were gram-positive rods, motile with a bundle of polar flagella, and formed subterminal oblong spores. The colonies in agar had an irregular shape with many platelike structures and were greyish white. Cellulose, xylan, and cellobiose served as substrates for growth. Acetate, propionate, butyrate, isobutyrate, isovalerate, lactate, succinate, H2, and CO2 were products of cellobiose fermentation. The optimal temperature and pH for growth were 35 degrees C and 7, respectively. The DNA composition was 40 mol% G + C. The name Clostridium aldrichii sp. nov. is proposed. The type strain is P-1 (= OGI 112, = ATCC 49358).