Many of the Firmicutes bacteria responsible for plant polysaccharide degradation in Nature produce a multiprotein complex called a cellulosome, which co-ordinates glycoside hydrolase assembly, bacterial adhesion to substrate and polysaccharide hydrolysis. Cellulosomal proteins possess a dockerin module, which mediates their attachment to the scaffoldin protein via its interaction with cohesin modules, and only glycoside hydrolases and other carbohydrate active enzymes were known to reside within the cellulosome. We show here with Clostridium thermocellum ATCC 27405 that members of the serpin superfamily of serine proteinase inhibitors, which are best recognized for their conformational flexibility and co-ordination of key regulatory functions in multicellular eukaryotes, also reside within the cellulosome. These studies are the first to expand the cellulosome paradigm of protein complex assembly beyond glycoside hydrolase and carbohydrate active enzymes, and to include a newly identified functionality in the Firmicutes.