The nucleotide sequence of the Clostridium thermocellum gene bglA, coding for the thermostable beta-glucosidase A, has been determined. The coding region of 1344 bp was identified by comparison with the N-terminal amino acid squence of recombinant beta-glucosidase A purified from Escherichia coli. The deduced amino acid sequence corresponds to a protein of 51,482 Da. The coding region is flanked by putative promoter and transcription terminator sequences. The protein is unrelated to beta-glucosidase B of C. thermocellum, but has a high level of similarity with other bacterial beta-glucosidases and phospho-beta-glucosidases. Similarity is also observed with the beta-galactosidase of the archaebacterium Sulfolobus solfataricus. Unexpectedly, it was found that human lactase-phlorizin hydrolase contains three copies of a sequence closely related to C. thermocellum beta-glucosidase A (up to 40% sequence identity). These diverse beta-glucosidases can therefore be grouped into an enzyme family (BGA) of common structural design. Sequence comparison by hydrophobic cluster analysis revealed that all BGA enzymes share a well conserved region which is homologous to the catalytic domain of the widely distributed cellulase family A. A distinctive feature of this domain is the sequence motif His-Asn-Glu-Pro in which the catalytic residues His and Glu are separated by 35-55 amino acid residues. The cellulase family A and the beta-glucosidase family BGA might thus be considered as members of a protein super-family comprising beta-glucanases and beta-glycosidases from all three primary kingdoms of living organisms.