Cellulases hydrolyze cellulose, a major component of plant cell walls, to oligosaccharides and monosaccharides. Several Clostridium species secrete multi-enzyme complexes (cellulosomes) containing cellulases. C. thermocellum CelT, a family 9 cellulase, lacks the accessory module(s) necessary for activity, unlike most other family 9 cellulases. Therefore, characterization of the CelT structure is essential in order to understand its catalytic mechanism. In this article the crystal structure of free CelTdoc, the catalytic domain of CelT, is reported at 2.1 Å resolution. Its structure differs in several aspects from those of other family 9 cellulases. CelTdoc contains an additional -helix, -helices of increased length and two additional surface-exposed -strands. It also contains three calcium ions instead of one as found in C. cellulolyticum Cel9M. CelTdoc also has two flexible loops at the open end of its active-site cleft. Movement of these loops probably allows the substrate to access the active site. CelT is stable over a wide range of pH and temperature conditions, suggesting that CelT could be used to convert cellulose biomass into biofuel.