The effect of diazinon on some biochemical characteristics of Chilo suppressalis Walker (Lepidoptera: Pyralidae), rice striped stem borerAbstract : Rice striped stem borer is a cosmopolitan pest that widely distributed in different regions of northen Iran and spraying with diazinon is the most common procedure to control this pest. We evaluated comparative physiological effects of diazinon spraying on the three populations of Chilo suppresalis collected from Gourabzarmikh (Go), Sheikhmahale (Sh) and Rasht (Ra) in Iran. The determination of physiological effects of diazinon, activity levels of some enzymes such as aspartate aminotransferase, alanin aminotransferase, a-amylase, ATPase and lactate dehydrogenase was evaluated. Results showed that aspartate aminotransferase and alanin aminotransferase didn't have significant difference. Activity level of a-amylase, ATPase, LDH also demonstrated significantly different so that Go population was at a higher activity level of these enzyme than the other two populations. Glucose and protein amount showed a significant difference and their amount in Go population was higher than Sh and Ra populations. It is inferred that diazinon treatment decreased activity levels of several enzymes, ATPases, LDH and a-amylase, that had a significant effect on metabolism of nutrients in insect's body. This parameter could be used instead of mortality percent for efficiency of chemicals that would decrease times of spraying and prevent resistance to insecticides, environmental pollution and non-targent toxicity.