Genetic algorithms (GAs) coupling neural networks (NNs) were developed for the medium optimisation of xylitol fermentation by Candida mogii with high yield. The concentrations of 6 medium components were optimised within 50 experiments and 40 emulate experiments to give a maximum yield with minimum use of medium content. With the optimised medium composition in a stirred tank bioreactor for batch production of xylitol, under the same operating conditions, xylitol yield was improved by 15%, compared to results with the medium used previously. (C) 2002 Elsevier Science Ltd. All rights reserved.