A parasitologal investigation on the freshwater fishes of Northwest Iran was done between Summer 2003 to Summer 2004. Altogether 266 fish specimens have been collected from seven different stations of Aras, Zangbar and Ghotor rivers. They are as follows: 166 Capoeta capoeta, 15 Alburnus bipunctatus, 9 Lucioperca lucioperca, 12 Carassius carassius, 2 Barbus capito, 18 Rutilus rutilus, 24 Abramis brama, 7 Cyprinus carpio, 11 Aspius aspius, 2 Silurus glanis, and one Lueciscus cephalus. The fishes were transported alive to the laboratory where they weighted and measured and then killed by transitioned the spinal cords and then examined for different parasites. During in study, 7 different monogeneans were detected; Dactylogyrus extensus, D. chramuli, D. lenkorani, D. kendalanicus, Silurodiscoids siluri, Diplozoon megan, and Gyrodoctilus varicorhini. According to the results of this study Diplozoon megan is the first time recorded from the Caspian Sea Fauna Region. It is found in Capoeta capoeta, Alburnus bipunctatus, Rutilus rutilus, Abramis brama, Carassius carassius, Cyprinus carpio. There are also three new hosts for the first time is recorded for four different monogeneans from fresh water fishes of Iran.