Sucrose and maltose are commonly used sugars in brewing, baking and/or ethanol production industries. Since both sugars have been shown to be transported directly into the yeast cells of Saccharomyces cerevisiae (for maltose, it is the only mode of utilization), the way the two transport systems interact when the two disaccharides are present in mixture was investigated using both fermentation and radiometric studies. Two strains of S. cerevisiae were used in this study : 248 UNSW 703100 (laboratory strain) and All (industrial strain). The yeast cells were adapted on the disaccharides for up to 14 d before fermentation or radiometric studies were carried out. In batch culture studies, the adapted cells on either maltose for 2 d or sucrose for 14 d were grown on mixtures of sucrose and maltose containing different proportions of the two disaccharides. In both strains, the growth on the mixture was diauxic with sucrose and its hydrolysis products being utilized first. In radiometric studies, the affinity constant for maltose in maltose-adapted cells of strain 248 UNSW 703100 increased from 4.3 mM when the cells were incubated with [U-C-14] maltose to 19.3 mM when incubated with equimolar mixture of [U-C-14] maltose and unlabelled sucrose. The apparent affinity constant for sucrose transport in sucrose-adapted cells increased from 6.4 mM when the cells were incubated with [U-C-14] sucrose to 18.9 mM when incubated with equimolar mixture of [U-C-14] sucrose and unlabelled maltose.