Pretreatmentwith the microsomal oxidase inhibitor piperonyl butoxide (pb) greatly improved the topical insecticidal activity of the GABA antagonist, 4-n-propyl-2,6,7-trioxa-lphosphabicyclo[2.2.2]octane 1-oxide (4-n-propyl BP),* against the housefly, Musca domestica L., when the synergistic ratio was 313. The insecticidal activity was also enhanced by bypassing the integument with injection without pb pretreatment.When tested by four different methods, the order of the 24-hr LD50 values (ug/fly) for 4-n-propyl BP was: injection, pretreated with pb (0.169)