Diplozoon paradoxum is a flatworm (platyhelminth) from the class Monogenea found in freshwater fishes in Asia and Europe and known for its complete monogamy. This parasite is commonly found on the gills of European cyprinid fishes. It is usually around 0.7 centimeters long (approximately the size of a fingernail) and has bilateral symmetry. It has several hooks at its mouth which it uses to grab on to the gills of a fish. From there it feeds on the blood of a Cyprinid.