Varying amounts of viable Saccharomyces cerevisiae and Candida shehatae cells were coimmobilized in a composite structure consisting of a flat agar layer encompassed between two microporous membrane filters. This immobilized cell system was placed between the two halves of a two-chambered bioreactor and tested for ethanol production from a glucose-xylose mixture. One chamber of the bioreactor was filled with the sugar mixture whereas the second compartment received an equal volume of a substrate-free mineral medium. Batch incubation of the immobilized cell structure was performed at 30 degrees C under two different oxygenation conditions : limited aerobiosis in both bioreactor chambers (symmetrical aeration) and asymmetrical oxygen supply obtained by continuous gas bubbling ii? the two bioreactor compartments, i.e., anaerobiosis in the substrate chamber (N-2 bubbling) and microaerobiosis in the buffer chamber (bubbling of a O-2/N-2 mixture at low O-2 content). Pure cultures and cocultures of S. cerevisiae and C. shehatae were able to simultaneously consume glucose and xylose, producing ethanol and xylitol. The fermentation efficiency of immobilized cell mixtures was no higher than that of pure cultures, however. Asymmetrical oxygenation conditions affected rite fermentation performance of immobilized cultures. Coimmobilized S. cerevisiae and C. shehatae both displayed growth inside the composite structure. Biofilm-like gel areas of high cell density appeared behind the microporous membrane filters during incubation of immobilized organisms. The cell number and repartition of the two yeasts in the gel layer depended on their relative proportions in the initial cell loading and the oxygenation conditions of the structure.