Xylitol production by Candida boidinii NRRL Y-17213 was investigated in fed-batch fermentations with xylose (50, 100 g litre(-1)) and a mixture of glucose (25 g litre(-1)) and xylose (25 g litre(-1)). All fermentations were initially batch processes with high levels of aeration and rapid production of biomass. Faster growth occurred when a mixture of glucose and xylose, instead of xylose, was used as a substrate; glucose was assimilated first and maximal xylitol production was 39.41 g litre(-1), compared with 46.5 and 59.3 g litre(-1) in the processes with xylose alone. Fed-batch cultures were characterized with higher xylitol yields (0.57-0.68 g g(-1)) and production rates (0.32-0.46 g litre(-1) h(-1)) compared with a batch process. Ethanol was accumulated in all processes, in smaller quantities, varying from II to 21% of the xylitol concentration. The fed-batch process with the highest initial xylose concentration (100 g litre(-1)) and the lowest level of aeration in the first phase, resulted in the highest yield of xylitol (75% of theoretical) and the highest productivity (approximately 35% higher than the productivities of the other two fed-batch experiments).