The gene of the monomeric multimetal beta-galactosidase of Saccharopolyspora rectivirgula was cloned and sequenced. Although the enzyme could be assigned as a member of beta-galactosidases belonging to the glycosyl hydrolase family 2, it has unusual structural features for beta-galactosidase of this family; it contained a unique sequence which consists of approximately 200 amino acid residues with no similarity to known proteins. This 200-residue sequence exists as if it is inserted into a sequence homologous to the active-site domain of the Escherichia coli lacZ enzyme.