Class Monogenea is a monophyletic grouping of the phylum Platyhelminthes. Monogenea are the only ectoparasitic flatworms of the phylum, compared to their sister groups, Trematoda and Cestoda, which are both endoparasitic. Unlike both phyla, Monogenea have a simple life cycle with no intermediate host; over the course of a life cycle, they will only attach to their definitive host, which tends to be an aquatic vertebrate such as a marine fish. Adult Monogenea latch onto the epidermis of their hosts with suckers and hosts.
Like the class Trematoda, Monogenea are commonly referred to as flukes.