The behavioral response of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.), to moisture gradients has been studied in some detail, but the effect of carbon dioxide (CO₂), alone and in combination with low humidity, has never been reported. The effect of CO₂ is of some interest because CO₂ levels in stored grain are often elevated well above normal atmospheric levels by the metabolism of insects, fungi, and the grain itself. Also, CO₂ is sometimes applied to bulk grain to control stored-product insects. The response of adult male O. surinamensis to humidity alternatives of 60 or 100% RH was determined in 2-way choice chambers after various treatments. Beetles removed from laboratory cultures and held at 60% RH with food for 4 h showed a strong avoidance of the higher humidity (hygronegative response). Beetles held without food in a dry atmosphere (<5% RH) with an elevated level of CO₂ (39%) for 4 h showed a reversal of the response from hygronegative to hygropositive. The same period of exposure, again without food, to dry air (<5% RH), or to a moist atmosphere (80% RH) with an elevated level of CO₂ did not produce a reversal. The failure of CO₂ to produce a reversal when exposure occurred in a moist atmosphere suggests that the gas acts indirectly on the humidity response by increasing the rate of moisture loss. Elevated levels of CO₂ are known to cause prolonged opening of the spiracular regulatory apparatus that controls moisture loss in insects.