BACKGROUND: Members of the genus Gyrodactylus species are one of the extensive groups of monogenean ectoparasites which parasitize marine, brackish and freshwater fishes. OBJECTIVES: The main objective of the present study is the identification and diagnosis of the Gyrodactylus specsies on the fishes in certain rivers in the southwest of the Caspian Sea basin. METHODS: The field investigations were carried out from March 2015 to June 2016 and approximately, 1240 fish specimens belonging to two families, nine genera and nine species from Babolrud, Tajan, Shirud, Tonekabon, Nekarud, Siahrud, Telar and Haraz rivers were examined. RESULTS: At least eleven Gyrodactylus species were isolated from skin and gills of the examined fishes. Among them, Gyrodactylus mutabilitas, G. sprostonae and G. prostae have been previously reported to be found in Iran. The rest, including Gyrodactylus ctenopharyngodonis, G. gobioninum, G. katharineri, G. nemachili, G. proterorhini, G. ophiocephali, G. varicorhini and G. vimbi are reported for the first time from Iranian Fishes. A tissue-specific microenvironment is seen in infection with Grodactylus proterorhini which was isolated from the gills, but mostly from the surface of the skin of Neogobius pallasi which is mainly specific to the brackish water of estuaries and coastal area. CONCLUSIONS: The host spectrum of Gyrodactylus species in Iran is actually wider and the further research may demonstrate that classification of individual species in terms of their host specificity will have to be changed and additional species of Gyrodactylus species would be found.