The continuous production of extracellular heterologous beta-galactosidase by a recombinant flocculating Saccharomyces cerevisiae, expressing the lacA gene (coding for beta-galactosidase) of Aspergillus niger was investigated. A continuous operation was run in a 6.5 1 airlift bioreactor with a concentric draft tube using lactose as substrate. Data on the operation with semi-synthetic medium with 50 and 100 g/l initial lactose concentrations are presented. The best result for beta-galactosidase productivity-6.2 x 10(5) U/l h-was obtained for a system operating at 0.24 h(-1) dilution rate and for a lactose concentration in the feed of 50 g/l. This value represents a 11-fold increase in beta-galactosidase productivity when compared to batch culture. Together with extracellular beta-galactosidase production an ethanol productivity of 9 g/l h was obtained for the bioreactor fed with 50 g/l initial lactose concentration at 0.45 h(-1) dilution rate. In addition to beta-galactosidase and ethanol production, this system allowed for complete lactose metabolism. The feasibility and advantages of using continuous high-cell-density systems operating with flocculent yeast cells for extracellular protein production is clearly shown. (C) 2004 Elsevier Ltd. All rights reserved.