The gas-actuated thermometer is formed as a unitary, monolithic glass structure with a gas bulb at one end and an indicator tube extending therefrom. The bulb and tube are permanently and hermetically sealed to contain the actuating gas therein. A closed cell foam indicator resides in the indicator tube. The indicator travels along the tube in accordance with the expansion and contraction of the gas within the bulb and adjacent portion of the tube. A second gas or mechanical spring is provided in the opposite end of the tube to balance the pressure of the indicator gas. The indicator tube, and preferably also the bulb, are flattened to an elliptical or similar cross-sectional shape. This facilitates the fit of the device for use as an oral medical thermometer, and the wider cross section enables both Celsius and Fahrenheit temperature scales to be marked along the indicator tube.