The insecticide activity of piperine, cinamoil amide, and tetrahydropiperine against Lucilia cuprina and Musca domestica adults were conducted at the Federal Rural University of Rio de Janeiro. Chemicals were topically applied on thoracic areas of the flies and the toxicity was determined after 24 and 48 hr post treatment and LD50 were calculated using Probit Analysis. Tetrahydropiperine (THP amide) was the only substance which demonstrated insecticide activity against both species of flies. LD50 against L. cuprina and M. domestica were 16.25 and 7.65 microg/fly, respectively, after 24 hr of treatment. Similar results were observed after 48 hr post treatment because the LD50's were 18.03 and 6.57 microg/fly, respectively. Males of L. cuprina were more resistant to tetrahydropiperine than females. However females of M. domestica were more resistant to the insecticide than males.