Megaclausia mirabilis n.gen., n.sp. a new copepod commensal with the maldanid polychaete Rhodine gracilior is described from material collected in the Firth of Forth and off the Northumberland coast. The species is a member of the family Clausiidae and is readily distinguished by the unusually large size of the female and by its leg structure. Other genera of the family are briefly reviewed and the position of Megaclausia is discussed.