Two phosphorolytic enzymes displaying activity towards the soluble cellulose degradation products cellobiose and cellodextrins were purified from the crude extract of the cellulolytic thermophile Clostridium stercorarium. Both phosphorylases have monomeric structures with molecular masses of 93 and 91 kDa, respectively. Although the N-terminal amino acid sequences are highly similar, a clear distinction of the two enzymes could be made on the basis of their substrate specificities: the enzyme designated cellobiose phosphorylase cleaved exclusively the disaccharide substrate, whereas the enzyme designated cellodextrin phosphorylase accepted only oligosaccharides as substrates. Kinetic constants were determined for the cleavage of cellobiose and cellodextrins. Maximal activity was observed at 65 degrees C in the pH range 6.0-7.0 for both enzymes. The sequences of the genes cepA and cepB encoding the cellobiose phosphorylase and the cellodextrin phosphorylase, respectively, have been submitted to the GenBank database.