Physiological studies of Saccharomyces cerevisiae strains harboring the MEL1 gene were carried out in aerobic batch cultivations on glucose-galactose mixtures and on the disaccharide melibiose, which is hydrolyzed by the enzyme melibiase (Mel1, EC 3.2.1.22) into a glucose and a galactose moiety. The strains examined (T200, T256, M24, and TH1) were all derived from the bakers＇ and distillers＇ strain of S. cerevisiae, DGI 342. All the strains showed a significant higher ethanol yield when growing on glucose, and half the biomass yield, compared with growth on galactose. The maximum specific uptake rates were 2.5-3.3-fold higher on glucose than on galactose for all the strains examined, and hence, ethanol production was pronounced on glucose due to respiro-fermentative metabolism. The T256 strain and the T200 strain having the MEL1 gene inserted in the HXK2 locus and the LEU2 locus, respectively, hydrolyzed melibiose with low specific hydrolysis rates of 0.03 C-mol/g/h and 0.04 C-mol/g/h, respectively. This resulted in high biomass yields on melibiose in the order of 10 g/C-mol compared with 3.7 g/C-mol for M24 and 1.6 g/C-mol for TH1. The M24 strain, constructed by classical breeding, and the mig1/gal80 disrupted and melibiase-producing strain TH1, were superior in their ability to hydrolyze melibiose into glucose and galactose showing specific melibiose hydrolysis rates of 0.17 C-mol/g/h and 0.24 C-mol/g/h, respectively. Hence, high ethanol yields on melibiose were obtained with these two strains. Growth on the glucose-galactose mixtures showed a reduction of glucose control successfully obtained in the M24 strain and the TH1 strain. (C) 2000 John Wiley & Sons, Inc.