Adsorbing carriers for immobilisation of Candida guilliermondii cells to use for xylitol production from sugarcane bagasse hemicellulose hydrolysate were tested. Biomass was immobilised in situ by natural adsorption, i.e. through direct contact between cells for the inoculum and carrier particles at the beginning of fermentations. The carriers employed were: Syrane porous glass beads with 2.53 mm diameter in average and pore diameter in the range 60-300 mum, supplied by Bioengineering (Wald, Switzerland), and NaX zeolite UOP WE 894 purchased from Plury Quimica S.A. (Diadema, SP, Brazil). At the end of the run with free cells taken as a reference test, xylitol concentration (P-f) achieved 35.5 g/l, corresponding to a xylose-to-xylitol yield factor (Y-P/S) of 0.72 g/g and a volumetric productivity (Q(P)) of 0.49 g/l h, while final cell concentration (X-f) and productivity (Q(x)) were only 5.32 g/l and 0.048 g/l h, respectively. Both systems with immobilised cells exhibited lower xylitol productions (P-f = 28.8-29.5 g/l, Y-P/S = 0.52-0.53 g/g, Q(p) = 0.32-0.33 g/l h) and higher cell growth, with particular concern to porous glass (X-f = 10.5 g/l, Q(x) = 0.10 g/1h). Electronicmicroscopy observations demonstrated that the excellent performance of porous glass as cell support was due to the development of a thick extracellular matrix either within the large pores or on the surface of this material. As a consequence, almost 50\% of the cells resulted to be adsorbed to the carrier at the end of the run. This growth was also responsible for a decrease in the fraction of xylose available for xylitol production. Employing zeolite, a material with pore size smaller than cell size, immobilised cells represented only 30\% of the final population and immobilisation was just observed on the carrier surface. The low cell attachment on this material can be explained by the stress exerted on the outer immobilised cells by the friction among beads. (C) 2004 Elsevier B.V. All rights reserved.