The dosage-mortality curves of CH3Br, methyl formate, ethylene dichloride, and CCl4 for the red flour beetle, Tribolium castaneum, at 30[degree]C and 2 hrs.'' exposure are presented. CH3Br and methyl formate are much more toxic to T. castaneum than ethylene dichloride and CCl4. The latter 2 fumigants are nearly equal in toxicity to T. castaneum at the 50% point. CCl4 was mixed with CH3Br, methyl formate, and ethylene dichloride in equitoxic units in several proportions, and the dosage-mortality curves for these mixtures are presented. Criteria for the detn. of antagonism and synergism are stated. Antagonism is present in the 1:1 and 1:3 methyl formate CCh and 1:1 and 1:3 ethylene dichloride-CCl4 mixtures. In general, a decrease in toxicity to T. castaneum occurs when CCh is mixed with CH3Br, methyl formate, or ethylene dichloride: in no case is there evidence of an increase in toxicity; in the majority of cases there is either antagonism, or a slight decrease in toxicity.