From 1,760 freshwater threespine stickleback examined, plerocercoids of Schistocephalus solidus and parasitic copepods, Thersitina gasterostei, were found on 22% and 21%, respectively. The former was more common in pond fish (29%) than stream fish (4%), and conversely the latter was more common in stream fish (56%) than pond fish (7%). Fish infested with S. solidus were hindered in speed and agility; those with T. gasterostei had hypertrophied gill filaments.