In this study, the knockdown and mortality effects of imidacloprid and methomyl were investigated. The residual surface applications were carried out to determine the knockdown effects (KDt(50) and KDt(95)) and mortality (LD50 and LD95) induced by each insecticide. For mortality comparisons, the susceptible house fly ( Musca domestica L., Diptera: Muscidae) of a WHO population and three natural field-collected M. domestica populations from Turkey were used. In conclusion, it was found that the resistance to imidacloprid and methomyl was significantly higher in the field populations when compared to the susceptible population from WHO. The results showed that applicators and pest management decision-makers should control and conduct an integrated pest management strategy by including biological agents to prevent the development of high levels of resistance in the field populations. Journal of Vector Ecology 35 (1): 144-148. 2010.