The beta-D-glucosidase I (EC 3.2.1.21) activity of Bifidobacterium breve 203 isolated from feces of adult humans was found to be very low, but when B. breve 203 was acclimated to cellobiose, the activity of the enzyme increased in the acclimated cell, B. breve 203 clb. With the aim of elucidating the mechanism, responsible for this increased activity the translation and transcription of the BD-glucosidase I gene were investigated using an antibody against beta-D-glucosidase I and a probe specific to the gene, respectively. Western and Northern blots showed marked differences in the expression of the enzyme and the level of its mRNA between B. breve 203 and B. breve 203 clb. However, genomic Southern blots revealed only insignificant differences in the copy numbers of the open reading frames in the two strains. The 5’ banking regions of the beta-D-glucosidase I gene of B. breve 203 and that of B. breve 203 clb were therefore analyzed. The transcriptional start point, located 94 bp upstream from the first methionine codon, was identified by primer extension. A sequence, -TTGGAA-(15 bp)-TAATCT-, located 8 bp upstream from the transcriptional start point, was assigned as the promoter of the gene. A sequence highly homologous to the lac operator region of Escherichia coil was found downstream of the transcriptional start point.