Farmers stock peanuts were treated with primiphosmethyl (0-(2-(diethylamino)-6-methyl-4-pyrimidinyl 0,0-dimethyl phophororthioate)) applied at rates of 10, 20, and 30 ppm as a protectant against stored-product insects and stored in metal bins (4. 5 metric tons) for 1 year. Similar peanuts were treated with malathion (diethyl mercaptosuccinate S-ester with 0, 0-dimethyl phosphorodithioate applied at a rate of 52.1 ppm as a standard for comparison. Although residues of pirimiphos-methyl decreased ca. 63per thousand during the year (half of the decrease during the first 4 months), rates of 20 and 30 ppm gave excellent protection for 1 year, and a rate of 10 ppm gave protection for about 6 months. Malathion was relatively ineffective, either because it degraded so rapidly the first 2 months or because malathion-resitant strains of insects were present. Of the 16 species of stored-product insects found in the peanuts, red flour beetles, Tribolium castaneum (Herbst), and almond moths, Cadra cautella (Walker), were the predominant species.