Xylitol was produced using xylose as the carbon source in a fluidized bed reactor containing Candida guilliermondii FTI 20037 immobilized on porous glass. The reactor was loaded with 200 g of immobilized system and operated at a volume of 1.41 in semicontinuous mode. The fermentation was performed in seven cycles (total time of 672 h), using fresh medium for each one. The first and the second cycles provided the highest values of xylitol yield (0.79 and 0.57 g/g, respectively), and volumetric productivity (0.52 and 0.60 g/(1 h), respectively), a decrease being observed afterwards. An increased number of cycles should result in better system performance, due to cell adaptation to the medium. However, cell wash-out occurred between cycles, causing the cell-particles to become unsatured. As a result, part of the xylose was used to produce biomass rather than xylitol. (C) 2002 Elsevier Science Ltd. All rights reserved.