A ${eta}-galactosidase$ gene of Bifidobacterium adolescentis Int57 (INT57) was cloned using the shotgun method. The sequence of the ${eta}-galactosidase$ gene existing in the sequenced 3,260-bp fragment showed higher than 40% homology with other bacterial ${eta}-galactosidase$ genes. The expression in Escherichia coli suggested that the ${eta}-galactosidase$ might have a monomeric, dimeric, or tetrameric protein structure. This is probably the first peer-reviewed sequence analysis of the ${eta}-galactosidase$ gene of the genus Bifidobacterium.