Although freshwater turtles and their epibiotic hosts are a good model for studying epibiotic interactions (including basking, the effects of turtle species, and geographic variation), information on diatom-turtle relationships are sparse, primarily documenting diatoms on two turtle species. The objective of this study was to characterize diatoms on freshwater turtles by comparing assemblages across: 1) four species: the common snapping turtle (Chelydra serpentina), the false map turtle (Graptemys pseudogeographica), the eastern mud turtle (Kinosternon subrubrum) and the common musk turtle (Sternotherus odoratus) and 2) the spatial range of the common snapping turtle (from Oklahoma, Arkansas, Illinois, Wisconsin, and New York). Turtle specimens came from museum collections at the Sam Noble Oklahoma Museum of Natural History and The Field Museum in Illinois. Six standardized areas on the turtle carapace were sampled for diatoms. Diatom assemblages were significantly different across all turtle species. Luticola cf. goeppertiana occurred on 97% of Oklahoma turtles and abundance differed with the following pattern: false map turtles > common snapping turtles > common musk turtles > eastern mud turtles. Diatom assemblages on common snapping turtles were different across states (OK ≠ IL, WI, NY, with AR intermediate). Luticola cf. goeppertiana occurred on 84% of sampled common snapping turtles and Oklahoma turtles had a higher mean abundance than the other four states. Observations of Luticola taxa, including Luticola cf. goeppertiana on turtles in both the northern and southern hemisphere indicates that this genus occurs on a variety of turtle species, and the species of Luticola found on turtles differs spatially. This research shows a new use of museum specimens that allows efficient data collection and prevents unnecessary collection of live turtles.