The tomato whitefly, Bemisia tabaci (Genn.) (Hemiptera: Aleyrodidae) is a worldwide pest of many crops. Organophosphorous (OP) insecticides have been used to control this insect for many years and are still being used despite the development of resistance. In this work' evaluation of the biological activity of three organophosphorous insecticides, profenofos 72% E.C. (Selecron); chlorpyrifos 48% E.C.(Dursban) and fenitrothion 50% E.C. (Sumithion)  on the felid strain of tomato whitefly was carried out on tomato plants.  Whitefly adults were collected from four different Egyptian Governorates throughout three successive years (2016-2018). The LC₅₀ value of each compound in each Governorate was used to determine the biochemical responses of this insect pest.The colorimetric test of esterase activity is a useful tool to detect the level of resistance against this group of insecticides. The obtained results showed that profenofos insecticide was more toxic on the tested insects followed by chlorpyrifos, while fenitrothion showed a low toxic effect on the tested insects in all years. The activities of α and β esterases have fluctuated up and down in some Governorates.