The house fly, Musca domestica L. (Diptera: Muscidae) is known as a mechanical vector of many pathogenic bacteria, fungi and parasites. The use of synthetic pyrethroid (SP) insecticides is recommended by the World Health Organization for the control of adult houseflies. Despite the fact that SPs have been used effectively, this insect has developed resistance in many regions/countries of the world. The use of synergistic substances to overcome insecticide resistance is widespread. Piperonyl butoxide (PBO) is used as a synergist in combination with SPs for the control of houseflies. The aim of this research was to determine the effect of SPs (cypermethrin, permethrin and deltamethrin) and their combination with different rates of PBO (1: 0.125, 1:0.25, 1:0.5 and 1:1) against four Turkish house fly populations (Ankara, Antalya, Gaziantep and Sanhurfa) in laboratory conditions. According to results the use of PBO in combination with insecticide active substances causes an increase in the mortality rates after 24 h. Also the toxicities of tested chemicals, based on their KDT50 (Knock-down 50 times) values, increased when mixed with PBO.