Resistance mechanisms to oxydemeton-methyl in Tetranychus urticae Koch (Acari: Tetranychidae)Abstract : The resistance mechanisms to oxydemeton-methyl were surveyed in two Iranian strains of the two spotted spider mite, Tetranychus urticae Koch. Bioassay was carried out on two strains, collected from Tehran and Rasht using a dipping method. The results of bioassay indicated that resistance ratio was 20.47 for resistant strain. The activity of esterase and glutathione S¬ transferase in resistant and susceptible strains showed that one of resistance mechanisms to oxydemeton-methyl was esterase-based resistance and glutathione S-transferase. The esterase activity of the resistant strain was 2.5 and 2.14-fold higher than those of the susceptible strain for α-naphtyl acetate (α-NA) and β-naphtyl acetate (β-NA) respectively. The kinetic characteristics acetylcholinesterase (AChE) showed that the AChE of resistant strain had lower affinity to artificial substrates; acetylthiocholine and butyrylthiocholine than that of susceptible strain. I50 of oxydemeton-methyl for resistant and susceptible strains were 2.68×10 M and 7.79×10 M respectively. The results suggested that AChE of resistant is insensitive to oxydemeton-methyl and ratio of AChE insensitivity of resistant to susceptible strain were 3.49 and 7.8-fold to oxydemeton-methyl and paraoxon, respectively.