The faunal inventory of the Great Plains state of South Dakota includes 15 families, 40 genera, and 77 species of Ephemeroptera, including the first published state records for 37 species, and 187 new county records for 64 of the species. The mayfly fauna of South Dakota includes transcontinental species, widespread species, many species that typify the central lowlands of North America, and some 18 species that find their eastern range limits in South Dakota, five species that find their northern range limits in South Dakota, and 16 species that find their western range limits in South Dakota and often additional states directly north or south of South Dakota.