We investigated the behavior of yeast of the genus Kluyveromyces (K. fragilis 507, K. lactis 29 and K. lactis 10), which grow on lactose as sole carbon source, since they possess an enzyme system for the utilization of this sugar. We determined the beta-galactosidase activity of these strains, grown in the logarithmic phase in media containing glucose and lactose. On addition of 0 to 12% v/v ethanol to cells treated with toluene, we did not observe inhibition of the enzyme in strain 10 of Kluyveromyces lactis, which showed the greatest activity (704.4 Units). Since there exist the possibility of industrial utilization of concentrated whey (4 times), we performed fermentation tests of the three strains, at 30 C, in media containing initial lactose concentrations of 16.5 and 24.5%. After 48 h the residual lactose concentration was practically zero, and the ethanol concentrations had reached 7.60 and 10.10% v/v. It might be expected that the rate of fermentation of a disaccharide such as lactose would be related to the rate of hydrolysis of the sugar, so that strains having a higher rate of enzymatic hydrolysis should show a higher fermentation rate. However, we did not observe such behavior, as strains of Kluyveromyces having enzymatic activities as different as K. lactis 10 (704.4 U) and K. lactis 29 (189.7 U) did not show any great difference in the production of ethanol from lactose.