Natural history collections provide a wealth of information beyond the scope of their initial purpose, including a historical window into host-parasite relationships. For example, collections can reveal historical infections of fishes by the freshwater gill-maggot Salmincola californiensis, a parasite of increasing concern because of negative effects on salmonids that use large reservoirs in western North America. Here, we examined 1,241 preserved specimens collected between 1933 and 2014 from the Willamette River Basin, Oregon and deposited in the Oregon State Ichthyology Collection. We confirmed the historical presence and broad distribution of freshwater gill-maggot species affecting Cutthroat Trout Oncorhynchus clarkii, Rainbow Trout O. mykiss, Chinook Salmon O. tshawytscha, and Mountain Whitefish Prosopium williamsoni. In particular, S. californiensis was found infecting Pacific trout and Chinook Salmon. In addition, a previously undocumented freshwater gill-maggot species was discovered on Mountain Whitefish. The ability to both detect historical infections as well as to identify previously undocumented parasites demonstrates the value of historical collections for modern explorations of fish infections and highlights a need to keep archival representatives in mind when collecting fish.