We investigated reports from commercial fishers of parasitized American eels (Anguilla rostrata) in the Patuxent River (located in the mid-Chesapeake Bay) and discovered that some eels were infected with an exotic swim bladder nematode, Anguillicola crassus. Here, we (1) describe the dispersal of this blood-feeding nematode, endemic to the Japanese eel (A. japonica), in Europe and North America; (2) discuss what is known of the effects of this parasite on eels; (3) summarize current knowledge of the life cycle and identification of this parasite; and (4) present new data on the range of A. crassus in the eastern United States. We examined the swim bladders of 329 American eels from 4 sites in the mid- and upper Chesapeake Bay as well as 150 eels from 4 sites in the Hudson River in New York. Prevalence ranged from 10% to 29% in the Chesapeake Bay and from 0% to 12% in the Hudson River. Maximum intensity of infection was 24 for Chesapeake Bay eels and 3 for Hudson River eels. We urge our U.S and Ca...