Xylanase activity of Clostridium thermocellum, an anaerobic thermophillic celluloytic bacterium, was characterized. The activity was localized both in the cellulosome (the principal multienzyme, cellulose-solubilizing protein complex) and in noncellulosomal fractions. Each of these fractions contained at least four major polypeptide bands which contributed to the xylanolytic activity. In both cases, pH and temperature optima, product pattern, and other features of the xylanase activity were almost identical. The main difference was in the averaging molecular weights of the respective polypeptides which appeared responsible for the activity. In the noncellulosomal fraction, xylanases with M(r)s ranging from 30,000 to 65,000 were detected. Distinct from these were the cellulosomal xylanases, which exhibited much larger M(r)s (up to 170,000). The cellulosome-associated xylanases corresponded to known cellulosomal subunits, some of which also exhibited endoglucanase activity, and others which coincided with subunits which appeared to express exoglucanaselike activity. In contrast, the noncellulosomal xylanases hydrolyzed xylan exclusively. β-Glucosidase and β-xylosidase activities were shown to be the action of different enzymes; both were associated exclusively with the cell and were not components of the cellulosome. Despite the lack of growth on and utilization of xyland or its degradation products, C. thermocellum produces a highly developed xylanolytic apparatus which is interlinked with its cellulase system.