A sonic gas analyzer intended primarily for estimation of CO2in the physiological range of 0–10% is described. The instrument uses a resonant cavity maintained in oscillation by a transistor amplifier as the measuring device; the change in frequency caused by the presence of CO2in the cavity is linearly related to the amount of CO2present. As described the instrument is intended for continuous sampling at a rate of about 2 l/min. The time of response is about 0.5 sec for full deflection.