Plerocercoids of the cestode Schistocephalus solidus are reported for the first time from the body cavity of anadromous threespine stickleback inhabiting Mud Lake, Alaska. Most infected stickleback harbored a single large plerocerciod (mean weight  =  0.447 g, range  =  0.228–0.716 g). The overall prevalence of plerocercoids across genders and 2 yr of samples was 1.4%, but prevalence was significantly greater in males than in females. Because of the large size of the plerocercoids, anadromous stickleback were probably infected as juveniles before leaving the lake, suggesting that plerocercoids can live in the body cavity of oceanic stickleback for several years.