Gyrodactylus mediotorus King, Marcogliese, Forest, McLaughlin and Bentzen, 2013, previously described from the spottail shiner Notropis hudsonius (Clinton) in the St. Lawrence River, Quebec, was identified from weed shiner Notropis texanus (Girard) in Wisconsin. The parasite was primarily observed to infect the fins and, to a lesser extent, the skin. BLASTn searches of a 436 bp partial 18S rRNA gene and 1066 bp targeting the ITS region were highly similar (100%; 98.75%, respectively) to Canadian specimens of G. mediotorus. The slight genetic difference coupled with nearly identical morphology led us to conclude the Wisconsin specimens on weed shiner were a variant of G. mediotorus. This species also shares a prominent anteromedial knob on the ventral bar (and noticeable similarity in overall form of the haptoral hard parts and male copulatory organ) with G. campostomae Wellborn, 1967, G. laruei Kritsky and Mizelle, 1968, G. protuberus Rogers and Wellborn, 1965, G. rhinichthius Wood and Mizelle, 1957, G. spathulatus Mueller, 1936 and G. stunkardi Kritsky and Mizelle, 1968. It seems likely these knob-bearing species are members of a gyrodactylid lineage that has radiated among leuciscid and catostomid host fishes endemic to North America, although whether this knob is a shared or independent character remains unsolved.