The gene encoding beta-galactosidase of Lactococcus lactis ssp. lactis ATCC7962 was cloned and its nucleotide sequence was determined. The beta-galactosidase of L. lactis was expressed in Escherichia coli and transformants containing this gene fragment appeared as blue colonies on LB plates containing X-gal. The beta-galactosidase activity of E. coli transformant was thirty times higher than that of L. lactis. The gene for the 115 kDa beta-galactosidase has a 2991-bp open reading frame preceded by a putative ribosome binding site. The deduced amino acid sequence show a high degree of homology to the beta-galactosidase of E. coli, and the putative active site residues are conserved (Glu-429 and Tyr-475).