Recently there has been increased concern about hazard to crop workers who enter fields or orchards following application of pesticides. In past years one of the more toxic organophosphorus pesticides, parathion (O,O-diethyl O-p-nitrophenyl phosphorothioate) has, on rare occasions, caused illness in workers exposed to residues during hand fruit thinning operations in apple orchards in the State of Washington (QUINBY and LEMMON 1958). The purpose of the present study was to determine if there is any increase in residue levels of parathion or more toxic conversion products as a result of repeated weekly applications to apple and peach trees. Alterations of parathion by photochemical, hydrolytic, and metabolic processes might lead to increased hazard to crop workers who have intimate contact with treated foliage.