The amperometric carbon dioxide sensor we developed uses chemoautotrophic thermophilic bacteria and an oxygen-sensing electrode. The sensor was fabricated by packing the sensitive area of the bicarbonate sensor in a cell with a gas-permeable membrane on one side. The membrane was constantly supplied with an oxygen-saturated buffer solution. The sensor's operating range was 34°C to 58°C. A linear relationship was obtained in a 1-mM to 8-mM NaHCO3concentration in a buffer solution (pH 5.5) and a 3% to 12% CO2concentration in air. The effect of the sensor's low sensitivity to sodium acetate and ethanol was greatly improved by supplying fresh, oxygen-saturated buffer solution. The sensor had a life of more than a month.