Bucephalidae is a family of trematode flatworms that parasitize fish. They lack suckers, having instead a muscular organ called a "rhynchus" at the front end which they use to attach to their hosts. The characteristics of the rhynchus are used to help define the genera of the family. It is one of the largest digenean families, with 25 genera containing hundreds of described species. Bucephalids are cosmopolitan, having been recorded all over the world. They are parasites of fish from freshwater, marine, and brackish water habitat types.