Stored wheat treated with14C‐chlorpyrifos‐methyl and14C‐malathion has been shown to contain residues which are not extractable by conventional analytical procedures. Grain containing such residues was bioassayed using the stored product insect T. castaneum. There was no effect on the insects in the tests involving the non‐extractable residues of malathion, but there was a significant reduction in the number of adults reaching maturity on chlorpyrifos‐methyl treated extracted grain. This effect could be associated with the trichloropyridinol metabolite of chlorpyrifos‐methyl which has been shown to comprise 60% of the non‐extractable residue.