Direct electron-transfer-type bioelectrocatalytic oxidation Of D-gluconate is studied with D-gluconate dehydrogenase (GADH, EC 1.1.99.3, from Gluconobacter frateurii) at indium tin oxide electrodes. The adsorbed GADH gives a surface-redox wave attributable to the heme c site in the absence of the substrate and clear catalytic oxidation current in the presence of the substrate. The enzyme kinetics, the surface electron transfer kinetics, and the adsorption characteristics of GADH have been analyzed by voltammetry and quartz crystal microbalance measurements.