A biosensor using pyrroloquinoline quinone-dependent aldose dehydrogenase (ALDH) as a biological component was developed and used for the measurement of the aldose sugars xylose and glucose. Different immobilization methods for ALDH in carbon paste were studied. The best electrode performance was obtained when ALDH was adsorbed on the surface of a carbon paste electrode. Several mediator compounds were mixed into the carbon paste. The lowest working potential and highest catalytic current were obtained with dimethylferrocene as a mediator. Both storage and operational stability of the ALDH electrodes could be improved by the application of a membrane consisting of a poly(ester-sulfonic acid) cation-exchanger, Eastman AQ-29D. Application of the membrane reduced the non-specific oxidation of fermentation samples on the electrode surface.