The advantages of nanofiltration membranes coupled with a CSTR were demonstrated for the semicontinuous production of lactic acid from whey permeate. Lactic acid was removed from the growth medium while lactose was kept in the bioreactor with the bacterial cells; moreover, Mg2+ ions were also recycled in the bioreactor at 96% and the nanofiltrate color was greatly reduced The highest volumetric productivity achieved with this device was 7.1 g l(-1) h(-1) and the lactate concentration was 55 g l(-1). The specific productivity was 3.54 h(-1). More than 99% of the membrane fouling after 44 h of fermentation was reversible. The initial permeate flux was restored easily by a water rinse. The performance of this type of membrane bioreactor was discussed.