The substrate-oxidizing activity of sake spoilage lactic acid bacteria (hiochi bacteria) was measured using a benzoquinone-mixed carbon paste electrode on which the bacteria had been immobilized. The electrode immersed in a buffer began to produce an anodic current following the addition of D-glucose or gluconate, reflecting the substrate-oxidizing activity of the immobilized bacteria. The time course of the first derivative of the current (dI/dt) exhibited a maximum, the magnitude of which increased with an increase in the number of hiochi bacterial cells immobilized on the electrode. When heat-treated cells were immobilized, the value at the maximum (dI/dt(max)) was lower than that obtained with the electrode on which cells which had not been heat-treated had been immobilized, The decrease in dI/dt(max) increased at higher heating temperatures and longer heating time. The relative decrease in dI/dt(max) due to heat treatment was smaller than the relative decrease in the number of colony formation due to the same treatment, suggesting that cells injured by heat treatment still retain substrate-oxidizing activity.