Arowana (Osteoglossum bicirrhosum) and cat fish (Hypostomus plecostomus) are two species of ornamental fishes which are mostly imported to Iran. Monogenea are plathyhelminthes which can dominantly infest the fish skin and gills with high host specificity. In this research, 35 O. bicirrhosum and 50 H. plecostomus with clinical signs were collected from different ornamental fish markets in Semnan, Iran and transferred alive to the Lab. Fish skin and gills were examined for monogenean infestations. No monogenea was observed on the skin specimens, but Gonocleithrum cursitans and Unilatus unilatus infested 54.28% and 74% of gills in O. bicirrhosum and H. plecostomus, respectively. This is the first report of both G.cursitans and U. unilatus in Iran.