The endoparasitic nematode Hysterothylacium brachyurum was described from tissues of Micropterus dolomieu (Smallmouth Bass) in Lake St. Clair, Michigan (Ward and Magath, 1917). It has been found in 9 species of fishes in Lake Huron (Bangham 1955). Literature searches and lists in Hoffman (1967) indicate that H. brachyurum is widespread in eastern North America from Ontario to Texas, in many species of freshwater fishes. It was found in four species of introduced fishes in the Sacramento-San Joaquin Delta: Ameiurus catus, White Catfish; Morone saxatilis (Striped Bass); Pomoxis nigromaculatus (Black Crappie); Alosa sapidissima (American Shad). Two of these species, A. sapidissima and M. saxatilis are anadromous, so that this parasite may occur in these fishes in brackish and marine regions of the estuary.