＇Lactic yeasts＇ produce some extracellular metabolites, as well as biomass, when cultivated in whey. Mixed cultures are able to use several sources of carbon simultaneously and therefore the use of mixed yeast cultures is proposed in order to reduce the whey COD and increase the biomass yield. Of the mixed yeast cultures tested, the highest biomass yield was obtained with Torulopsis cremoris and Candida utilis. C. utilis consumed some metabolic by products generated by T. cremoris. Repeated fed-batch culture of T. cremoris and C. utilis, carried out in an airlift bioreactor designed to operate variable Volume processes, is a potential alternative for the treatment of whey, since it produces a high yield of biomass (0.75 g of biomass/g of lactose) and a greater COD removal efficiency (95.8%) than those reported in the literature. (C) 2000 Elsevier Science Ltd. All rights reserved.