Head space volatiles, including 73% di-n-propyl disulfide, were collected from freshly crushed neem seeds. This compound along with previously reported diallyl disulfide (di-2-propenyl disulfide) were toxic when applied topically or as a fumigant to Tribolium castaneum adults and 8-, 12-, and 16-d-old larvae, and Sitophilus oryzae adults. Di-n-propyl disulfide significantly decreased the growth rate and dietary utilization with moderate inhibition of food consumption in both insects. The total coefficient of deterrence for this compound ranged between 68.5 and 178.6, which suggests that it has medium to very good deterrent activity vis-à-vis the treatment concentration and instar. Di-n-propyl disulfide and diallyl disulfide presented a similar effect on efficiency of conversion of ingested food, which is reduced 3-fold; this implies that both compounds are physiological toxicants. Present studies clearly demonstrate that di-n-propyl disulfide could be a potent toxicant, fumigant, and feeding deterrent for stored grain pests, if a suitable formulation and application procedure are developed.