Three modified atmospheres with different CO₂ concentrations (30, 40, and 50%) were tested against larvae and adults of the red flour beetle, Tribolium castaneum (Herbst), at 25 and 35 °C by using several exposure periods to detect the mortality percentages and lethal time (LT) values. The effect of CO₂ under pressure (5, 10, and 15 bars) on the target insect was also evaluated at 25 °C and two exposure periods. Larval and adult mortality of 100% after four days at 35 °C was obtained with the modified atmosphere containing 50% CO₂. The LT₅₀ for adults was 6.8 days at 30% CO₂ concentration and 25 °C and decreased to 1.8 days with 50% CO₂ concentration at 35 °C, whereas larval LT₅₀ was 3.9 and 0.8 days at the same CO₂ concentrations and temperatures, respectively. At the highest tested CO₂ concentration, significant larval sensitivity was observed after the first two days of exposure. Using CO₂ under 15 bar pressure decreased the necessary exposure period to attain 100% mortality of adults and larvae from several days at normal pressure experiment to five hours.