Dichlorvos vapor was applied weekly from dichlorvos- impregnated resin pellets into 42 5 cubic m (1500 cu ft) chambers. The chambers contained insect-free flour packaged in cotton sheeting, and in multiwall paper bags stacked at various distances from similar bags of infested flour. The dichlorvos concentration attained in the chambers from the weekly application of vapor was about 3 micron g/l. After 5 months of storage, the number of insects found in the flour indicated that the dichlorvos applications retarded the infestation of flour in cotton bags and prevented the infestation of flour in the multiwall paper bags. Chemical analysis showed that dichlorvos residues were less than 0 1 ppm in the flour and, in general less than l l micron g/dm squared (10 micron g/ft squared) on the packaging materials.