Kluyveromyces lactis and Kluyveromyces marxianus are industrial yeasts widely used in the production of the beta-galactosidase enzyme. Biosynthesis of beta-galactosicase is controlled by glucose repression. In this study it was demonstrated that the derepression of beta-galactosicase biosynthesis in these yeast strains is inhibited by high osmotic stress. It was found that the beta-galactosiclase activity of K. lactis and K. marxianus remained approximately at the repressed level when these yeast cells were subjected to NaCl-, KCl-, or sucrose-induced high osmotic stress. Derepression of beta-galactosidase biosynthesis seems to be more sensitive to high osmotic stress in K. marxianus than in K. lactis. In addition, it was shown that nitrogen starvation resulted in a significant decrease in the level of beta-galactosidase biosynthesis in K. lactis, while nitrogen starvation led to a 2-fold increase in beta-galactosidase biosynthesis in K. marxianus. Results of this study indicated that high levels of NaCl, but not sucrose, inhibited the growth of K. lactis and K. marxianus.