The only treatment approved for use on the outer surface of insect-resistant packages is a coating of pyrethins synergized by piperonyl butoxide. These insecticides may migrate through the packaging materials into the stored commodity. Laboratory tests evaluated the effectiveness of various packaging materials as barriers to the migration of piperonyl butoxide from treated kraft paper into flour. During 6 months of exposure, polyester, cellulose triacetate. polyvinyl fluoride, polyvinyl alcohol, and some polyvinyl chloride films and sarancoated kraft, saran-coated paperboard, and vinyl-coated cellophane prevented the migration of deleclable quantities of piperonyl butoxide. Thirty-four other materials including greaseproof and glassine papers, saran-coated kraft, saran-coated polyethylene, cellulose acetate butyrale and polycarbonate films, and some cellophanes, greatly reduced but did not prevent migration of the piperonyl butoxide.