When 14C-radiolabeled malathion was applied topically to susceptible and malathion-resistant Rhyzopertha dominica and degradation monitored over the subsequent 24-hr period three fractions of metabolites were observed; the carboxyesterase products, a group of polar compounds, and a pool of unextractable residues. Overall slightly more metabolism occurred in the susceptible strain but in only one instance did the recovered metabolites exceed 10% of the applied dose, of which 30 to 50% remained as intact malathion and was deposited in two phases on to the filter papers on which the insects were confined. The first phase, occurring during the first 3 hr after treatment, was observed in both the susceptible and resistant strain and was apparently due to physical removal of malathion by the insects. The second phase occurred only in the resistant strain, between 6 and 24 hr after treatment, and was the result of the insects excreting intact malathion. It would appear that this contributes significantly to the resistance mechanism in this strain of malathion-resistant R. dominica.