When diazinon sprays and dusts were applied to wheat at 1, 2.5, and 5 ppm and stored for 168 days in open jars at ambient room temperature, the toxicity of the treated grain to insects was markedly reduced after 7 days. Granary weevil adults, Sitophilus granaries (L.), were considerably more resistant than the red flour beetle, Tribolium castaneum (Herbst), or the saw-toothed grain beetle, Oryzaephilus surinamensis (L.). The red flour beetle was most susceptible. The dusts were more effective than the sprays and the efficacy of both was reduced more in the 13.4±0.5% moisture wheat as compared with the wheat maintained at 10.6±0.6%. Chemical estimations indicated considerable loss of dizainon immediately after treatment. The dusts generally persisted in greater amounts than the sprays. The residues from wheat within 168 days varied from 0.29 ppm from samples receiving an initial application of 1 ppm to 2.47 ppm from an initial treatment of 5 ppm. Quality evaluations on milling, baking, and flavor indicated no untoward effects when wheat was treated with 8 ppm diazinon and evaluated at a maximum of 9 days later.