en Acute Toxicity and Behavioral Changes Associated with Diazinon in Rutilus rutilus caspicus and Hypophthalmicthys molitrix عمومى General پژوهشي Research <b>Background:</b> Diazinon is an organophosphorous pesticide which widely found in municipal, agricultural, and urban storm water discharges. The present study was conducted to achieve lethal concentration (LC50) and behavioral changes of Rutilus rutilus caspius and <b>Hypophthal-micthys molitrix after exposure to lethal concentration of diazinon. <br>Methods: </b>The experiment was carried out in static conditions, based on instructions of OECD in 4 days under controlled water physicochemical conditions with pH of 7.2±0.2, oxygen of 7±0.3 mg/l, total hardness of 180 mg CaCo3 and temperature of 24±1 C°. All fishes were accli-matized in 400 L aquaria for 10 days. Treated aquaria had concentrations of 0.5, 1, 2, 4, 8, 10, 20, 40, 60, and 80 ppm of diazinon for H. molitrix, and 1, 2, 4, 8, 10, and 20 for R. rutilus caspi-cus, while there was no toxic concentration for the control group. LC1, LC10, LC30, LC50, LC70, LC90, and LC99 were calculated for 24, 48, 72, and 96 hours. <br><b>Results: </b>LC50 96h diazinon values were 3.93 and 1.71 ppm for H. molitrix and R. rutilus caspi-cus, respectively. Clinical observation revealed that the poisoned fishes suffered from nerve paralysis syndrome. The fishes exhibited irregular, erratic, and darting swimming movements, severe aching, and collapse to the bottom of the aquarium. <br><b>Conclusion: </b>These findings suggest that diazinon has medium toxicity at low concentrations for thede two species and causes morbidities. Diazinon, Lethal Toxicity, Organ-Phosphorus Pesticide, Roach, Silver Carp. Diazinon, Lethal Toxicity, Organ-Phosphorus Pesticide, Roach, Silver Carp. 1354 1359 http://ijt.arakmu.ac.ir/browse.php?a_code=A-10-225-5&slc_lang=en&sid=1 Seyed Ali Akbar Hedayati سیدعلی اکبر هدایتی 10031947532846004556 10031947532846004556 No Department of Fisheries, Gorgan University of Agricultural and Natural Resources, Gorgan, Iran. Department of Fisheries, Gorgan University of Agricultural and Natural Resources, Gorgan, Iran. Hamed Ghafari Farsani حامد غفاری فارسانی 10031947532846004557 10031947532846004557 No MD, Young Researchers and Elite Club, Shahrekord Branch, Islamic Azad University, Shahrekord, Iran. MD, Young Researchers and Elite Club, Shahrekord Branch, Islamic Azad University, Shahrekord, Iran. Saeid Shahbazi Naserabad سعید شهبازی ناصرآباد 10031947532846004558 10031947532846004558 No MD, Young Researchers and Elite Club, Yasuj Branch, Islamic Azad University, Yasuj, Iran. MD, Young Researchers and Elite Club, Yasuj Branch, Islamic Azad University, Yasuj, Iran. Mohammad Hasan Gerami محمد حسن گرامی m.h.gerami@gonbad.ac.ir 10031947532846004559 10031947532846004559 Yes PhD Candidate, Young Researchers and Elite Club, Shiraz Branch, Islamic Azad University, Shiraz, Iran. PhD Candidate, Young Researchers and Elite Club, Shiraz Branch, Islamic Azad University, Shiraz, Iran.