Cellulases are o-glycosyl hydrolases (GH) that hydrolyze the β-1,4 glucosidic linkages of cellulose. The cellulase system is grouped into “glycoside hydrolase (GH) family” classified by different means, according to their substrate specificities, their reaction, their mechanisms or their structural similarities. The cellulase complex contains three basic components that can be present either as a single polypeptide or grouped together into a multienzyme complex called a cellulosome. The cellulase system is composed of three main classes based on their activity on a wide range of substrates. This is quite difficult, because enzymes have overlapping specificities toward substrates that are themselves poorly defined. The three main classes are: a) Endoglucanases (endo-1,4-β-glucanases or 1,4-β-D-glucan 4-glucanohydrolases EC 3.2.1.4) hydrolyze cellulose chains at random positions in less than crystal regions, to generate mainly short-chain oligosaccharides producing free ends with a rapid change in the degree of polymerization. The substrates are amorphous cellulose, such as carboxymethylcellulose, H3PO4 or alkali-swollen cellulose, instead of crystalline cellulose. 1.19) produce cellobiose by attacking the free ends of the chains. They are believed to work in a processive manner, that is, one enzyme molecule can release several units of cellobiose from the cellulose chain without leaving the substrate. c) β-glucosidases or cellobiases (β-D glucoside glucohydrolases, EC 3.2.1.21) are very important components. of the cellulase system in that they complete the hydrolysis to glucose of short-chain oligosaccharides and cellobiose which are released by the medium of paper ......id, acetic acid, formic acid, H₂ and CO₂ as fermentation products which increase the basic ecological, industrial and bioenergetic interests of this particularly thermophilic bacterial species. Thermophiles produce a number of enzymes, for example cellulases, xylanases, mannases, etc. which have many applications in various industrial processes including food, detergents, textiles, paper, tanneries. and the biorefining sectors. However, the use of cellulases in industries faces a major setback due to high production cost and low enzymatic activity. This has encouraged the search for new thermophilic cellulases with minimum cost and improved activity. With the aim of exploring new cellulolytic enzymes more suited to industrial needs, we have described here the cloning and high-level expression of the new β-glucosidase BglB, from Clostridium. thermocell in Escherichia coli.