The presence of organic chemical materials in surface water resources, besides having adverse effects on the consumer, reduces the desirability of aquatic fauna and even results in their complete annihilation. Determination of Diazinon residues in the Tajan River (TR) was traced in order to investigate the level of concentration of this pollutant from the agricultural activities around the river. This was accomplished by taking samples in 2008, starting in April at three-week intervals and continued up to October. After each sampling, the existing Diazinon concentration was determined in the laboratory using a high performance liquid chromatography (HPLC) instrument. The largest Diazinon concentration was 47 micrograms per liter found in June and at the third sampling station. Analysis of the results confirmed that the Diazinon concentration of Tajan River increases with the beginning agricultural activities in specific fields covered with rice. This Diazinon concentration appeared throughout summer at different concentrations. According to the criteria recommended by EPA for Diazinon pollutants to protect the river ecosystem, with a one-hour average concentration of 0.17 μg/lit which is not exceeded more than once every three years (acute toxicity criteria), it can be concluded that Diazinon concentration in the Tajan river affected agricultural activities and increased the risk to the river ecosystem.