Two of the major constituents of the essential oil of garlic, Allium sativum L., methyl allyl disulfide and diallyl trisulfide, were tested against Sitophilus zeamais Motschulsky and Tribolium castaneum (Herbst) for contact toxicity, fumigant toxicity, and antifeedant activity. The contact and fumigant toxicities of diallyl trisulfide were greater than that of methyl allyl disulfide to the adults of these two species of insects. These two compounds were also more toxic to T. castaneum adults than to S. zeamais adults. Older T. castaneum larvae were more susceptible to the contact toxicity of the two compounds, whereas younger larvae were more susceptible to the fumigant toxicity of these compounds. Both compounds reduced egg hatching of T. castaneum and subsequent emergence of progeny. Diallyl trisulfide totally suppressed egg hatching at 0.32 mg/cm², and larval and adult emergence at 0.08 mg/cm². Methyl allyl disulfide significantly decreased the growth rate, food consumption, and food utilization of adults of both insect species, with feeding deterrence indices of 44% at 6.08 mg/g food for S. zeamais and 1.52 mg/g food for T. castaneum. However, it did not affect any nutritional indices of T. castaneum larvae. Diallyl trisulfide significantly reduced all of the nutritional indices in all of the insects tested. Feeding deterrence indices of 27 and 51% were obtained in S. zeamais adults and T. castaneum larvae, respectively, at the concentration of 2.98 mg/g food, whereas feeding deterrence of 85% was achieved in T. castaneum adults at a much lower concentration of 0.75 mg/g food. Hence, diallyl trisulfide is a more potent contact toxicant, fumigant and feeding deterrent than methyl allyl disulfide.