The tomato leafminer, Tuta absoluta (Meyrick) (Lepidoptera: Gelechiidae), is a globally important economic insect pest of tomatoes that has rapidly expanded in range. The pest is generally controlled by insecticides; however, reduced susceptibility of T. absoluta to many recommended insecticides has been reported. The aims of this study were to determine the efficacy of diazinon, spinosad, abamectin, and Bt on second instar larvae of Iranian greenhouse populations of T. absoluta from Soufian (SF) and Qom (QM) and a susceptible (SS) population originally from Bonab, and assess the possibility of resistance in the populations.The results showed that spinosad was the most effective insecticide, whereas in contrast, Bt was ineffective against the populations. Abamectin was effective, but susceptibility in SF population was lower than in QM and SS. Susceptibility to diazinon was low in SF and QM populations. Mediation of diazinon resistance by metabolic mechanisms and activity of detoxifying enzymes and acetylcholinesterase on resistance were assessed. The results showed that enhanced activity of monooxygenases and esterases may have played a role in resistance of T. absoluta to diazinon. Our results will be useful in resistance management in this injurious pest and reduce the negative impacts of high rates of insecticide use on the environment.