Partial substitution of sugarcane molasses by cheese whey in the fed-batch production of baker's yeast was evaluated. A sugar feeding profile based in a commercial process and different modes of addition of beta -galactosidase were used. Molasses substitution of 46%, in terms of sugar fed to the bioreactor, was reached and no significant differences in biomass volumetric productivity, by-products yields, and baking quality were observed. However the biomass yield was 6% lower.