Clostridium thermocellum strains SS21 and SS22, fermented a variety of pure crystalline cellulose materials efficiently with a maximum ethanol yield (gram ethanol/gram substrate (g/g)) of 0.37 and 0.35, respectively at low substrate (8 g/litre) concentration. With 80 g/litre filter paper, the strain SS21 converted 71 g filter paper into 18.46 g ethanol and strain SS22 fermented 75.4 g filter paper into 20.36 g ethanol per litre of the medium. Both the strains grew poorly on crude biopolymers but fermented them easily after alkali treatment. When grown with 8 g/litre alkali extracted paddy straw (AEPS), alkali extracted sorghum stover (AESS) or alkali extracted deshelled corn cobs (AECC) the ethanol yields were similar to those obtained with purl cellulosic materials. At high substrate (100 g/litre) concentrations, AEPS served as the best substrate for ethanol production when compared with filter paper and other biopolymers. A maximum amount of 21.47 g and 23.57 g ethanol was produced from 82.6 g and 87.3 g AEPS per litre of medium by strains SS21 and SS22, respectively. This is the highest ethanol production among the strains of C. thermocellum so far reported.