Seven mayfly species represent new state records, and an additional eight mayfly species represent new county records for Missouri. These 15 species from eight families increased the total Ephemeroptera species richness of the state to 110. Many species reported here include habitat specialists and large-river species collected as either larvae or adults. Homoeoneuria ammophila (Spieth) was collected from the Missouri River, a species having otherwise sparse records in North America, and the predacious, widespread Pseudiron centralis (McDunnough) was collected from the Mississippi River following an absence in Missouri records for over 65 years. Ongoing threats from land-use change and climate alterations make aquatic insect distribution records critically important. Greater sampling efforts should be concentrated in large-river habitats.