Toxicity of Carbon dioxide (30%), Phosphine (100 ppm) and their combination against the developmental stages of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) were studied under two degrees of temperatures20 and 30±2 °C, inside steel bins. The residual impact of the combination on the treated wheat was evaluated with respect to histological changes in different organs of rat fed on the treated wheat. Lethal time for 50, 90 and 99% (LT₅₀, LT₉₀ and LT₉₉) after exposure periods of 1, 2, 3 and 5 days were calculated. Results indicated that the pupae were the least sensitive stage while the larvae were the most susceptible one to the combination. In addition, the mortalities of the different stages of T. castaneum in the combination treatment were higher than those obtained from each treatment alone. Histological examination for the rats feeding treated wheat with carbon dioxide-phosphine combination was done. Overall, testis, epididymis, stomach and spleen of the treated rats were not affected while liver; kidney and lung were affected through the appearance of mild histological changes. It may be concluded that this combination is good effective and less harmful in the case of control T. castaneum in the stored grains and recommended feeding of animals on these fumigated grains after 3 days from fumigation.