The beta-galactosidase (beta-Gal) gene from Lactobacillus plantarum C3.8 was cloned and expressed in Lactococcus lactis and Escherichia coli. Hybridization experiments indicated that the gene is located on a plasmid and is present in other strains of Lactobacillus plantarum. Its sequence is very similar to a Leuconostoc lactis beta-Gal gene. Expression of the gene, both in Lactobacillus plantarum and in Lactococcus lactis, was four-fold higher in cells growth in lactose compared to those grown in glucose. The presence of the beta-Gal gene in Lactococcus lactis allowed this bacterium to be efficient in clotting milk.