Application of pyrroloquinoline quinone (PQQ)-dependant aldose dehydrogenase (ALDH) for the amperometric determination of xylose and glucose is described. Different flow injection configurations were investigated to optimize the measurement. The highest current densities could be obtained when the enzyme was immobilized directly on a graphite electrode. The best stability was achieved with controlled-pore glass (CPG)-immobilized ALDH. ALDH-CPG was also successfully used for the measurement of real fermentation samples. The measurement of samples containing only xylose as a carbon source correlated very well with high-pressure liquid chromatographic measurement, the correlation coefficient and conversion factor between the methods being 0.98 and 0.96, respectively. Also samples containing both xylose and glucose could be measured satisfactorily when the measurement with ALDH was combined with separate glucose measurement.