An exotic parasite of anguillid species of eels, first noted in a single South Carolina American eel (Anguilla rostrata), now appears to be spreading into other regions of eastern North America. Adult stages of the nematode, Anguillicola crassus, inhabit the lumen of eel swimbladders, while third and fourth stage juveniles are found in the swim bladder wall. A total of 193 American eels were collected during spring 1998 from four Chesapeake Bay tributaries (the Susquehanna, Chester, and Sassafras rivers, and St. Jerome's Creek) and examined for the presence of juvenile and adult stages of A. crassus. Prevalence (% of eels infected) in the four sampling localities ranged from 61-82%, and infection intensity ranged from 0 to 52 worms per eel. Mean intensity (average number of worms per infected eel) in each tributary ranged from 2.9 to 8.9 worms per eel. Fishery scientists should be on the lookout for this potential source of eel debilitation or even mortality.