Cotton whitefly, Bemisia tabaci is an important polyphagous insect pest which has developed resistance to various insecticides worldwide. Neonicotinoid pesticides such as imidacloprid and thiacloprid are agonists of nicotinic acetylcholine receptors (nAChR). Six B. tabaci populations were collected from different regions of Iran. Using a phylogenetic tree constructed based on the CO? sequence the biotype of Minab population was determined as B biotype. Bioassays were performed using a leaf deep method and susceptibility of the populations to imidacloprid were assayed. The lethal concentration 50 values (LC 50 ) for different populations showed diffrences in the susceptibility of B. tabaci to imidacloprid (24.4 to 189.8 mg/L). Ahvaz population with the lowest LC 50 value (24.4 mg/L) was considered as the susceptible population. LC 50 values of Isfahan, Kashan, Gorgan and Minab populations were estimated as 136.91, 106.95, 141.09 and 68.31 mg/L. The resistance ratio of the populations ranged from 2.8 to 7.8 for imidacloprid. The synergistic effects of piperonylbutoxide (PBO), triphenyl phosphate (TPP) and diethyl maleate (DEM) were evaluated on the susceptible (Ahvaz) and resistant (Kashan) populations to determin the involvement of cytochrome P450-dependent monooxygenase, carboxylesterase and glutathione S-transferases, respectively, in the resistance development. Based on the results, PBO and TPP showed the highest synergistic ratios of 2 and 2.4 in the resistance population of Kashan, respectively. The activity of carboxylesterases, glutathione S-transferases and cytochrome P450s were also measured biochemically, in order to determine which enzymes may be involved in the neonicotinoid resistance. There was an increase in the activity of cytochrome P450s up to 3-fold in the resistant population. Possibly cytochrome P450-dependent monooxygenases and carboxylesterases are the enzyme systems responsible for neonicotinoid resistance in the tested populations of B. tabaci . Keywords : Bemisia tabaci , resistance of imidacloprid, Synergistic ratio, Enzymes assay.