Analysis of a carboxmethyl-cellulase clone isolated from the cDNA library of the ruminal fungus, Piromyces rhizinflata 2301, revealed that the clone encoded a polypeptide containing a cellulase catalytic domain, designated CelAcd. CelAcd was flanked by a 28-amino acid linker peptide at the N-terminus and linked to a dockerin domain by a. 7-amino acid linker at the C-terminus. CelAcd showed homology with the family 5 of glycosyl hydrolases. CelAcd plus the linker peptides at both termini (designated CelcdN 'C ') was expressed in Escherichia coil and the purified enzyme nias characterized. The feature of particular interest of CelcdN 'C ' was its bifunctional endo-and exo-glucanase activity, demonstrated by its ability: to hydrolyse carboxymethyl cellulose (CMC) and pNP-P-B-cellobioside. Furthermore, CelcdN 'C ' exhibited relatively high ability to degrade both microcrystalline Avicel and filter paper. CelcdN 'C ' also showed activity against barley beta -glucan, Lichenin and oat spelt xylan. The optimal activity; conditions for CelcdN 'C ' with CMC as the substrate were pH 5.5 and 50 degreesC Fifty percent of the enzyme activity was lost when CelcdN 'C ' was treated at 55 degreesC for 10 min. CelcdN 'C ' retained more than 10% enzymes activity after being heated under 90 degreesC for 10 min. Deletion of the N-terminal flanking linker of CelcdN 'C ' (the resulting protein was designated CelcdC ') did not alter the enzymatic function of the catalytic domain. However the thermal stability of CelcdC ' was dramatically reduced. We conclude that the N terminal flanking linker of CelAcd stabilizes the enzyme protein.