Fumigant toxicity of rosemary essential oil on abamectin-resistant vs. susceptible populations of two spotted spider mites, Tetranychus urticae Koch was studied under laboratory conditions. Results indicated that fumigant toxicity of rosemary essential oil on the susceptible population was 3.06 times higher than that of the resistant one. Accordingly, there was a slight cross resistance observed between abamectin and rosemary essential oil vapors. In addition, in this survey, the effects of LC50 of rosemary essential oil on detoxifying enzymes were investigated in two treatment periods of 24 and 48 hours on resistant and susceptible populations. Assessment of esterase activity indicated that the enzyme activity of resistant population did not decrease in the treatment period of 24 hours but decreased significantly in the treatment period of 48 hours compared with the treatment periods of 24 hours and control. In the case of susceptible population, the esterase activity in treatment periods of 24 hours decreased significantly, whereas the activity significantly increased in the treatment periods of 48 hours as compared with the 24-hour ones. Results of variance analysis of glutathione-S-transferases enzyme activity data indicated that the enzyme activites are not affected by the 24 and 48 hours of treatment with rosemary essential oil. No significant difference was observed between the level of monooxygenases of susceptible populations in the treatments vs. control. However, the level of this enzyme in the resistant populations decreased significantly through passage of time.