Thirty-seven streams within 15 Missouri State Parks were sampled for immature and adult Ephemeroptera, Plecoptera, and Trichoptera (EPT) during 2002 and early 2003. Seven mesohabitats were sampled: riffle, run, leafpack, pool, no flow organic, bank, and silt/mud. Mesohabitat associations are reported for each species collected. All totaled, 34,251 EPT larvae were collected, all specimens of which were identified to the lowest taxon possible. Of approximately 70,000 adult specimens collected by blacklight and vegetation sweeping, 10,342 were examined and identified to the lowest taxon possible. Fifteen species endemic to the Interior Highlands and two species on the Missouri Species of Conservation Concern Checklist were collected. This research revealed a total of 213 species in 99 genera and 37 families of EPT in the designated state parks, representing 65% of the EPT fauna known from Missouri. Mesohabitat associations are reported for all species of larvae collected. Richness of mayflies and stoneflies was significantly highest during the spring season whereas richness of caddisflies was significantly highest during the fall. Richness of all three orders was highest in the riffle mesohabitat, and lowest in the pool and no flow organic mesohabitats. Many distribution records are reported and Serratella sordida McDunnough (Ephemeroptera: Ephemerellidae) and Triaenodes perna Ross (Trichoptera: Leptoceridae) were collected in Missouri for the first time.