Dactylogyrus agolumaccessorius n. sp. is described from the bigmouth shiner, Notropis dorsalis (Agassiz), from the Prairie Region, U.S.A. Dactylogyrus agolumaccessorius appears most closely related to D. julieae Rogers, 1967, by possessing a copulatory accessory piece with the distal end in the shape of a shepherd's crook. Dactylogyrus agolumaccessorius differs from D. julieae most notably by possessing a much narrower male copulatory organ. The apparent close relationship of D. agolumaccessorius and D. julieae supports a close phylogenetic relationship between their hosts (N. dorsalis for D. agolumaccessorius, and N. amplamala Pera and Armbruster and N. buccatus (Cope) for D. julieae) previously indicated by studies of skeletal morphology, allozymes, and mitochondrial DNA, and coevolution from a common ancestor of these parasites and their respective hosts.