The variation in the efficacy of pirimiphos-methyl, neem, sesame, castor and soybean oil either alone against the adults of CTC-12, pH-1, Kano and Black strains of Tribolium castaneum (Herbst) and in combination of four ratios of pirimiphos-methyl and oil (1:1, 1:2, 1:5, 1:10) against CTC-12 strain of the beetle was evaluated by film residue method. The results showed that the LD50 of pirimiphos-methyl was 0.304, 0.355, 0.264 and 0.317 μg cm -2 for CTC-12, PH-1, Kano and Black strains, respectively. The same value was 56.58, 23.07, 26.70 and 35.60 μg cm -2 from neem oil; 11.71, 2.89, 44.71 and 7.97 μg cm -2 for sesame oil; 14.85, 9.48, 42.97 and 23.40 μg cm -2 for castor oil and 19.86, 20.95, 22.96 and 67.27 μg cm -2 for soybean oil in the same order of strains, respectively. All oils proved additive when combined with pirimiphos-methyl except neem and soybean oils which showed an antagonistic effect against the CTC-12 strain of T. castaneum at 1:1 ratio. The maximum synergism of pirimiphos-methyl was observed at 1:10 ratio having the highest co-toxicity coefficient value for neem oil (4908.53) followed by sesame oil (434.11), castor oil (295.24) and soybean oil (232.93).