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...