Piperonyl butoxide (PBO), a methylenedioxyphenyl compound, is primarily used as a synergist in combination with space spray, residual and admixture products for the control of insect pests in or around domestic and commercial premises, especially food preparation areas. Also, tetramethrin is known as a knockdown agent on target organism and it is generally used with piperonyl butoxide. In this study, effects of piperonyl butoxide and tetramethrin combinations on biological activities of synthetic pyrethroids, cypermethrin, deltamethrin, and permethrin against different housefly (Musca domestica Linnaeus, 1758) populations were evaluated. In addition, the biological efficiency of the insecticides used in the study, insecticide + PBO and insecticide + PBO + tetramethrin combinations, against the WHO standard sensitive housefly population and housefly populations collected from different parts of Turkey were compared.Results showed that PBO extensively promoted the ratio of knockdown and killing effect values of the insecticides. The results also indicated that PBO and PBO + tetramethrin combinations moderately reduced the knockdown effect times of all formulation in all housefly populations. The knockdown effect times were more decreased by insecticide + PBO + tetramethrin combinations than insecticides that are used alone and insecticide + PBO combinations. Pest control, insect control, synergism, Turkey insects, fight against insect vectors of disease