By crossing different Kluyveromyces lactis strains, the role of genes LAC1, LAC2 and of gene C were analyzed. These genes are involved in the biosynthesis of enzymes for the metabolism of lactose and galactose. They control the biosynthesis of the lactose and galactose transport, of beta-galactosidase and of the three enzymes of the Leloir pathway. The presence of at least one of the LAC gene is required for the biosynthesis to occur. The gene C seems to code for a negative factor which blocks the expression of the LAC1 and LAC2 genes in the absence of an inducer.