A series of continuous fermentations were carried out with a production strain of the yeast Saccharomyces cerevisiae in a membrane bioreactor. A membrane separation module composed of ultrafiltration tubular membranes retained all biomass in a fermentation zone of the bioreactor and allowed continuous removal of fermentation products into a cell-free permeate. In a system with total (100%) cell recycle the impact of fermentation conditions [dilution rate (0.03-0.3 h -l ) ; substrate concentration in the feed (50-300 g -l -l ) ; biomass concentration (depending on the experimental conditions)] was studied on the behaviour of the "immobilized" cell population and on ethanol formation.M a x i m u m ethanol productivity (15 g. 1-1. h -1) was attained at an ethanol concentration of 81 g-1 -1. The highest demands of cells for maintenance energy were found at the maximum feed substrate concentration (300 g-1-1) and at very low concentrations of cells in the broth.