A lactose-limited synthetic medium was used to analyse the dynamics of batch growth and beta-galactosidase production in the yeasts Kluyveromyces and Candida, underfully aerobic conditions. In shake flasks cultures, no significant differences were observed in the growth yields among 18 tested strains and most of them showed comparable enzyme formation. Five of these strains, K. lactis CBS 2360, CBS 683, NRRL 1118, K. marxianus NRRL 1109 and C. pseudotropicalis NCYC 744 were further studied in fermenter cultures. Metabolic behaviour was characteristic of the Crabtree-negative yeasts. Ethanol formation was absent or very low (less than 5% of the lactose metabolized). During the exponential growth phase typical fluxes for lactose and oxygen uptake were 3.0 and 13 to 15.5 mmol g-1 h-1 respectively. Enzyme levels were constant during the exponential phase and increased 1.3-1.8-fold during the brief deceleration phase.