Experiments were conducted to evaluate the degradation rate of malathion under typical on-farm drying and storage conditions. To quantify malathion residues, an accurate and inexpensive extraction method was developed, using ethyl acetate as the solvent. Malathion residues in grain were detected by gas chromatography, and showed a 99% rate of recovery;Malathion 6% dust and 57% emulsifiable concentrate (EC) were applied to cool wet corn (CWC), 20% moisture content (mc), prior to drying to 12% mc, or to either hot dry corn (HDC) or cool dry corn (CDC), dried to 12% mc. Treatments were dried at one of three drying temperatures, 21, 48, 71°C. Storage temperature effects were evaluated for all treatments with corn stored at 3°C for 4 months followed by storage at 16°C for 7 months;Malathion applied to CWC before drying was significantly degraded by the drying process. Grain treated after drying retained significantly more malathion than grain treated before drying. There was no difference in the percent malathion remaining for application to HDC or CDC. The degradation rates for all treatments throughout the storage period did not differ significantly;Bioassay results using two test species showed that biologically active residues were present throughout the entire storage period. These results show that either malathion formation can be applied immediately before or after drying at a wide range of temperatures and still remain efficacious. The dust formulation was more stable than the EC formulation when exposed to high temperature and grain moisture. Application rates should be increased to compensate for losses due to heat and moisture when malathion is applied to HDC or CWC;The effect of indigenous Penicillium spp. on the degradation rate of malathion was examined. Corn at 25% mc was either autoclaved to eliminate fungal populations or left non-autoclaved, dried to a final moisture of 12% at either 21°C or 71°C, cooled and treated with 57% EC malathion. Malathion degradation and fungal growth were evaluated during four months storage at 3°C followed by 10 months storage at 16°C. Under these conditions, the degradation rate of malathion was not significantly affected by Penicillium spp. and there was no significant interactions among fungal populations, drying temperatures and sterilization on the rate of malathion degradation.