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 stercoruriurn. 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°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.Keywords: phosphorylase ; cellobiose ; cellodextrin ; Clostridium; thermophilic.Enzymes capable of degrading cellobiose and cellodextrins are considered as essential components of microbial cellulolytic enzyme systems. They convert the cellobiose and cellodextrins formed during the enzymatic degradation of cellulose by the synergistic action of endoglucanases and exoglucanases to fermentable sugars. As cellulolytic enzymes are generally subject to product inhibition, an adequate level of cellobiose and cellodextrin removing enzymes is required for efficient breakdown of cellulose. The metabolism of soluble cellulose degradation products involves hydrolytic and phosphorolytic cleavage (Coughlan and Mayer, 1992). Hydrolysis is catalyzed by p-glucosidases releasing glucose from the disaccharide and the nonreducing ends of the oligosaccharides. Phosphorolysis is energetically advantageous and might constitute the primary route of cellobiose and cellodextrin utilization in particular in anaerobic environments. Cellobiose phosphorylase (cel1obiose:orthophosphate n-D-glUcoSyl-tranSferaSe) and cellodextrin phosphorylase