The European red mite, Panonychus ulmi is one of the most important pests in the apple orchards. This species was introduced to Iran by apple seedlings from Europe. Apple growers often use several acaricides for the P. ulmi control. Fenpropathrin is an insecticide/acaricide which has been used extensively against P.ulmi in apple orchards. The aim of the present study was to study the evolution of fenpropathrin resistance in field-collected populations of P. ulmi. The fenpropathrin toxicities to twelve populations collected from apple orchards of three provinces (Isfahan, East Azarbaijan, and West Azarbaijan) were determined by a leaf disc spray method using Potter spray tower. The results of bioassays showed that Shanin Dej population was the most susceptible (LC50= 20 mg a.i.L-1) to fenpropathrin, and the Marand (LC50= 2556), Maraqeh (LC50= 1805), and Mianeh2 (LC50= 1340) populations from East Azarbaijan were showed about 149, 90, and 71-fold resistance to fenpropathrin, respectively. Ahar, Semirom1, Semirom2, Khafr, Salmas, Urmia, Mahabad and Mianeh1 populations were not resistant to fenpropathrin when compared with the susceptible population (Shahin Dej). In addition, the Marand, Maraqeh, and Mianeh2 populations were showed the cross-resistance to fenvalerate, with resistance ratios of 21, >20, and 19-fold, respectively. Effects three synergists on the fenpropathrin resistant populations were also tested. PBO, TPP, and DEM enhanced slightly the toxicity of fenpropathrin in Marand, Maraqeh, and Mianeh2 populations, and the resistance ratios enhanced compared with Shahin Dej population. These results indicated that the detoxification maybe is not the main factor for the development of fenpropathrin resistance in the tested P. ulmi populations and probably target site insensitivity is a major mechanism of fenpropathrin resistance. A discriminating concentration of fenpropathrin, 80 mg a.i. L-1, was established based on the LC95 value of the Shahin Dej population. The results showed that Shahin Dej and Semirom2 populations were susceptible (more than 60% mortality), and Ahar, Khafr, Mahabad, Urmia, Mianeh1, Semirom1, and Salmas populations showed a moderate resistance to fenpropathrin (between 30% and 60% mortality), while Marand, Maraqeh, and Mianeh2 populations were highly resistant to fenpropathrin (less than 30% mortality) when the discriminating concentration of fenpropathrin was applied. We suggest that 80 mg a.i.L-1 fenpropathrin as a discriminating concentration is a suitable bioassay marker for differentiating between the susceptible and resistant European red mite populations to fenpropathrin.