The cellulosome, a highly elaborate extracellular multi-enzyme complex of cellulases and hemicellulases, is responsible for the efficient degradation of plant cell-wall carbohydrates by anaerobic microorganisms. Cohesin and dockerin recognition pairs are integral to the architecture of the cellulosome. Thus, type I cohesin:dockerins are important for attaching the modular enzymatic components to primary scaffoldins to form the cellulosome. In contrast, type II dockerins located in primary scaffoldins bind to anchoring scaffoldins, thus contributing to the cell-surface attachment of the entire complex. Since anchoring scaffoldins usually contain more than one type II cohesin, they contribute to the assembly of polycellulosomes. Acetivibrio cellulolyticus possesses an extremely complex cellulosome arrangement which is organized by a primary enzyme-binding scaffoldin (ScaA), two anchoring scaffoldins (ScaC and ScaD) and an unusual adaptor scaffoldin (ScaB). A ScaB dockerin mutated to inactivate one of the two putative cohesin-binding interfaces complexed with the ScaC cohesin from A. cellulolyticus has been purified and crystallized and data were collected from tetragonal and monoclinic crystal forms to resolutions of 1.5 and 6.0 Å, respectively.