Cypermethrine is one of the most dangerous poisons that are commonly used in agricultural purpose. The aims of the present study are to investigate acute effects of cybermethrine as potential additives of water ecosystems to assess mortality effects of these chemical products on main cultured warm-water fish of Iran. Common Carp (Cyprinus carpio) was exposed to the cypermethrine (0, 0.5, 2, 4, 8 and 16 ppm). LC50 was determined with probit analysis. The 96 h toxicity tests showed 100% mortality in 8 and 16 ppm, while in all treatments one mortality was observed. As it was found that LC50 of cypermethrine in common carp (1.91 ppm) was lower than that of other species, it means there is resistance of common carp in comparison with other species. Eventually, the findings of this study indicate that LC50 cypermethrine is more toxic to cultured fish and it should be moderately used for agricultural purpose