Many ecosystems have been under the continuous influx of both organic and inorganic xenobiotics, including organophosphorus compounds, as a result of intense agricultural and industrial activities. Biochemical biomarkers have been increasingly used in determining the effects of xenobiotics in studies aimed at assessing ecological risks on ecosystems. In this study, as a model organism, housefly (Musca domestica L.) samples collected from 48 different locations belonging to 16 provinces in Aegean and Mediterranean regions of Turkey were screened for their variation in glutathione S-transferase (GST), percent remaining acetylcholinesterase (AChE) and general esterase activities, as biomarkers of environmental pollution. In all Aegean populations, the percent remaining AChE activities were found much higher than that of the control WHO strain indicating heavy organophosphate usage in this region, and a resistance mechanism likely developed as AChE insensitivity. However, the results also showed that compared with Aegean region, the populations from Mediterranean Region are more sensitive to this group of insecticides on the base of this enzyme. GST activities were also much higher than that of the susceptible control in all, except Osmaniye, populations that is indicative of intense usage of xenobiotics in these regions. General esterase activity assays resulted in the preference of alpha-NA in higher proportions than beta-NA as substrate. Also the results of these assays can be interpreted as development of resistance on the basis of mutant ali-esterase hypothesis to organophosphate group of insecticides in some areas screened. The concurrence of the significant variations observed in housefly AChE, GST, and general esterase enzyme activities, with the different xenobiotic stress conditions of locations emphasize their suitability to be utilized as biomarkers in biomonitoring.