A beta-galactosidase gene from Lactobacillus sake coding for lactose hydrolysis was cloned and expressed in Escherichia coli. Chromosomal DNA from L. sake was partially digested with the restriction enzyme Sau3AI, and the 3-6 Kb fragment was ligated to the cloning vector pSP72 digested with BamHI. One E. coli transformant expressing beta-galactosidase was isolated on X-gal plates. It contained a plasmid with an insertion of approx. 4 Kb. The restriction map of the recombinant plasmid was constructed. The characteristics of the recombinant beta-galactosidase were compared with those of the wild type. The optima pH and temperature for both enzymes was 6.5 and 50 degrees C, respectively. Stability of the enzymes at different temperatures and activity on lactose were determined.