The cellodextrin phosphorylase (CDP) gene of Clostridium thermocellum was cloned and sequenced. The nucleotide sequence of the insert of a positive clone contained an open reading frame of 2940 bp encoding a polypeptide of 980 amino acid residues with a calculated molecular mass of 111,182 daltons. Escherichia coli cells harboring the plasmid for expression produced CDP protein accounting for 30% of the total cellular proteins with an activity of 59.9 units/ml culture, which corresponded to 0.93 mg/ml culture. The expressed CDP could be used to synthesize cellulase inhibitors as cellooligosaccharide analogues using glucose-1-phosphate as a glucose donor and 4-O-beta-D-glucopyranosyl-1-deoxynojirimycin or 6-O-beta-cellobiosyl-1-deoxynojirimycin as an acceptor.