A new host (Cottus cognatus) for adults of the acanthocephalan Leptorhynchoides thecatus (Linton) is reported. Of 176 slimy sculpins examined from eastern Lake Michigan, 93% were infected by L. thecatus. The number of L. thecatus infecting slimy sculpins was directly related to size of fish. Mean number of L. thecatus per fish was 17 for males, 10 for females, and 3 for immatures. One male was infected with 132 L. thecatus.