Cotton whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) is one of the most seriously damaging insect pests of agricultural products worldwide. It is highly polyphagous, damaging a broad range of food as well as
non-food crops through direct feeding, excretion of honeydew and transmission of over 100 plant viruses. In Iran, growers usually use insecticides as the main control agent of Bemisia tabaci which may lead to insecticide resistance in the pest. An investigation of the susceptibility of three populations of this pest, collected from Tabriz, Gorgan and Qom regions, to imidacloprid and amitraz was carried out. Bioassays were conducted using an adult leaf-dip method. The results revealed that Tabriz strain was more susceptible to imidacloprid than the other two strains. Resistance ratios for Gorgan and Qom strains relative to Tabriz strain were 2.6 and 2.68, respectively. Bioassay results with amitraz revealed that Tabriz strain was 1.44 and 1.47 fold less susceptible than Gorgan and Qom strains, respectively. Biochemical assays were conducted through on estimation of esterase activity using ?- naphthyl acetate and ?-naphthyl acetate substrates. The results of with ?-naphthyl acetate substrate showed that esterase activity in Gorgan and Qom strains were respectively 1.37 and 0.85 fold of Tabriz strain. Esterase activity was also investigated using page method. Different esterase banding patterns in Gorgan and Qom strains were detected for comparison with Tabriz strain.