Abstract 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 11 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).