Bucephalus mytili is a parasitic flatworm of the class Trematoda. It is a parasite of fish and a parasitic castrator of the mussel Mytilus edulis, where it destroys the mussel's gonads and causes the mussel to grow much larger than normal. The cercaria of B. mytili was described in 1935 occurring in Mytilus edulis in Wales. It is the sporocysts, which are long and tangled within the mollusk host's digestive gland, that cause parasitic castration of the host.