The toxicity of fumigants to insects in atmospheres of CO2 was found to vary considerably with different fumigants and with different species of insects. Of 5 fumigants, acrylonitrile, methyl bromide, a mixture of acrylonitrile and methyl bromide, phosphine, and hydrogen cyanide, the toxicities of acrylonitrile and acrylonitrile—methyl bromide mixture were increased most; concentrations of CO2 between 20 and 50% were most effective. The effectiveness of CO2 declined with drop in temperature so that no appreciable enhancement of toxicity occurred at 0°C.