Evaluation of insecticide resistance and biochemical mechanism in two populations of Eurygaster integriceps puton (Heteroptera: Scutelleridae)Abstract : Sunn pest (Eurygaster integriceps Puton) (Heteroptera: Scutelleridae), is one of the most serious insect pests of wheat and barley in Iran and in the neighboring countries. In Iran since 1950?s control of Sunn pest was mainly based on chemical control especially of DDT followed by organophosphorus insecticide such as fenitrothion (since 1970?s). In the current study susceptibility of the two geographic populations (Mazandaran and Tehran provinces) of the insect to fenitrothion was assessed using bioassays as well as enzymatic evaluations. Calculted LD50 values against adult insects of Tehran and Mazandaran populations were 0.19 and 0.08 µg insect-1, respectively. Residual assays against second-instar nymphs of the Sunn pest showed that LC50 value for Tehran strain was 1.71 ppm while LC50 value for Mazandaran strain was 0.77 ppm. These data indicated that Tehran colony is less susceptible to insecticide than the Mazandaran colony. These findings were further supported by determination of esterase activity and I50 values of the two populations. Esterase activity of female individuals of Tehran and Mazandaran colonies to 1-NA (alpha naphthylacetate) substrate was 1.16 and 0.68 ìM product/min/mg protein, respectively. The female individuals of Tehran population contained almost twice as much esterase activity as Mazandaran population. Analysis of variance showed that there were significant differences between esterase activity of male and female and between two populations (P < 0.05).