Swingleus ancistrus n. sp. (Monogenea: Gyrodactylidae) is described from the skin and fins of Fundulus heteroclitus from Canary Creek Marsh, Lewes, Delaware. Subsequent records from New Jersey, Maryland, Virginia, and North Carolina are reported. Swingleus ancistrus is differentiated from S. polyclithroides, the only other species of the genus, primarily by its size, haptoral morphology, host, and locality. Swingleus ancistrus is larger in almost every dimension and exhibits a distinct notch on the anterior border of the opisthaptor, and several features of the haptoral anatomy and peduncular bar are unique. The prevalence of infection in hosts collected from the type locality in August was 100%. The maximum number of S. ancistrus recovered from a single host was 134. The average intensity of infection was 24 S. ancistrus per host from the type locality.