Development of prebiotics and probiotics would be beneficial for human health, and several kinds of oligosaccharides have been commercially available recently, as food additives with a promoting effect of Bifidobacterial growth in gut. But few studies on regulation of oligosaccharide assimilation by the bacteria have been reported. In this report we have shown that utilization of cellobiose by Bifidobacterium breve 203 is under plural regulations: production of β-glucosidase I of the strain was repressed by glucose and induced by cellobiose, hardly assimilable oligosaccharide by this strain. While after long-term acclimation to cellobiose the strain came to be assimilable of cellobiose and a mutant showed higher β-glucosidase I activity than that of the parental strain constitutively, indicating both of the regulation have released in the acclimated mutant.