An initial checklist of mayflies (Ephemeroptera) of Iran compiled based on the literature data and new data from the north-western Iran reported only 46 species and 25 genera. Available literature data were often highly fragmented and referred to material limited to occasional collections with only several specimens examined. Moreover, many studies (mostly focusing on the water quality assessment based on macroinvertebrates) reported species or genera with distribution restricted to the Nearctic and Neotropic Region, suggesting that the authors used inappropriate determination keys. Most of valid records of mayflies were confined to the northern part of Iran, mostly Alborz Mts. To fill a significant gap in our knowledge on mayfly diversity, we started an extensive study covering the geographical and ecological gradients within Iran. Our field trips conducted in 2016–2018 set up a network of 234 localities to study the diversity and composition of mayfly assemblages in different types of aquatic habitats. The study and its first results are introduced in this conference presentation.