the important areas of this industry in Iran. The present study was carried out to survey onectoparasite fauna of cold water fish farms in Mazandaran Province (Iran) during a period of 12 months.For this aim 500 samples were obtained from 50 fish farms and after arriving to laboratory the parasiteexamination started immediately. From the total of 500 specimens, 182 fish (36.4%) were infected withectoparasites. Through the parasitic examinations five species of parasites were detected, namely:Ichthyophthirius multifillis, Trichodina sp., Chilodonella sp., Gyrodactylus sp. and Dactylogyrus sp. Theresults showed that, from total percentage (36.4%) of detection, 31.4% of detected parasites wereprotozoan (16.4% Ichthyophthirius multifillis, 14% Trichodina sp., and 1% Chilodonella sp.) and 5% weremonogenean trematodes (3% Dactylogyrus sp. and 2% Gyrodactylus sp.). The highest prevalence ofparasites in our study belonged to the Ichthyophthirius multifiliis causative agent of White Spot Disease.