In dual substrate (5 g glucose l(-1), 5 g xylose l(-1)) batch fermentation of L. lactis IO-1 a classic diauxie was observed. In batch fermentations (5 g xylose l(-1)) xylose isomerase activity was only detected in xylose grown cells. In mixed-substrate, carbon limited chemostat cultures (5 g glucose l(-1), 5 g xylose l(-1)) xylose utilisation was partially repressed by glucose at dilution rates above 0.01 h(-1) and completely repressed at 0.50 h(-1).