Studies were made to determine whether the disappearance rate of malathion residues on fruits and vegetables is affected by previous treatments with paraoxon, parathion, or malathion. Paraoxon applied to plums, tomatoes, and string beans after harvest clearly retarded the disappearance rate of post harvest residues of malathion. Application of parathion or malathion one or more times during the week before the post harvest treatment of string beans or the field treatment of New Zealand spinach had no detectable effects on the disappearance of malathion from the bean pods during storage or from spinach leaves in situ. It is evident that paraoxon retards the disappearance rate of malathion by inhibiting carboxyesterases, one of the enzymatic systems responsible for the decomposition of malathion in the plant tissues.