The recombinant CelS (rCelS), the most abundant catalytic subunit of the Clostridium thermocellum cellulosome, displayed typical exoglucanase characteristics, including (i) a preference for amorphous or crystalline cellulose over carboxymethyl cellulose, (ii) an inability to reduce the viscosity of a carboxymethyl cellulose solution, and (iii) the production of few bound reducing ends on the solid substrate. The hydrolysis products from crystalline cellulose were cellobiose and cellotriose at a ratio of 5:1. The rCelS activity on amorphous cellulose was optimal at 70°C and at pH 5 to 6. Its thermostability was increased by Ca2+. Sulfhydryl reagents had only a mild adverse effect on the rCelS activity. Cellotetraose was the smallest oligosaccharide substrate for rCelS, and the hydrolysis rate increased with the substrate chain length. Many of these properties were consistent with those of the cellulosome, indicating a key role for CelS.