Plerocercoids of Schistocephalus sp. (Diphyllobothriidae) were found in the body cavities of 2 (0.9%) of 209 slimy sculpins, Cottus cognatus, collected in September 2003 from Lake Michigan, south of Manistique, Michigan, U.S.A. Mean intensity was 1. The mean lengths and weights of these 2 thawed, relaxed plerocercoids were 67 and 72 mm and 427 and 554 mg, respectively. The number of segments in each plerocercoid was 113, and both plerocercoids showed upturned edges of the first segment. The identification of these plerocercoids to species is discussed, and the occurrence of Schistocephalus sp. in fish from the Great Lakes is summarized. This is the first report of Schistocephalus in slimy sculpin from Lake Michigan and the second report of Schistocephalus infecting fish from this lake.