The vegetable leafminer, Liriomyza sativae Blanchard, is a key pest of vegetable and ornamental crops worldwide and is mainly controlled by insecticides. The toxicity of fenpropathrin on adult flies of F1 generation of six Iranian populations of the pest was assessed using the residual glass vial method. The bioassay results showed significant differences in susceptibility to fenpropathrin among the populations tested. Resistance ratios ranged from 5.09 to 28.32. The resistant populations were reared in the greenhouse under fenpropathrin pressure for 14 mo and then susceptibility of both larvae and adults of these strains to fenpropathrin was evaluated. The resistance levels significantly increased in these populations. Further study demonstrated that diethyl maleate and triphenyl phosphate had no synergistic effects on fenpropathrin, however, this compound was significantly synergized by piperonyl butoxide in all tested populations. S,S,S-tributyl phosphorotrithioate also showed a slight synergism with fenpropathrin. Enzyme assays for general esterases, glutathione S-transferases and monooxygenases were performed on adults and larvae of L. sativae. The activity of glutathione s-transferase in different resistant and susceptible strains was not significantly different. The activity of esterases in resistant strains was higher than the susceptible strain. Monooxygenase levels also were higher in resistant strains compared with the susceptible strain. Synergism and biochemical assays revealed that esterases and monooxygenases have a major role in resistance of L. sativae to fenpropathrin.