Introduction Species belonging to the Ephemeroptera order are known as the oldest and most primitive winged insects, having existed since the time of the Carboniferous and Permian. They show a hemimetabolous development and spend most of their life in fresh waters as nymphs (Brittain and Sartori, 2003). Ephemeroptera nymphs play a very important role in the aquatic food chain, especially in secondary production, since a large majority of them are herbivores, feeding on detritus or algae, and they are present in almost all kinds of fresh waters throughout the year (Zelinka, 1984; Brittain and Sartori, 2003). However, they are very sensitive to water pollution and therefore have been used as bioindicator organisms in several biotic indexes and methods (Elliott et al., 1988; Moog et al., 1997). Since Ephemeroptera is known as one of the oldest insect orders and the species have some limitations on their distribution such as having a short life span, weak flight ability in the adult stage, and a fully aquatic nymphal stage, they are one of the most important groups used in zoogeographical studies (Brittain, 1982; Kazancı, 2001a, 2001b). The Gediz River basin has an important location due to its very close geographical position to a migration route between Europe and Anatolia since the glacial age. Therefore, the aim of this study is to determine the Ephemeroptera fauna of the Gediz River basin and contribute to the knowledge of the Ephemeroptera fauna of Turkey. 2. Materials and methods A total of 3801 Ephemeroptera nymph specimens were collected from different parts and habitats of the Gediz River basin by sieves (pore size 1, 1.4, and 2 mm) and hand net between July 2011 and October 2012 (Figure 1). Specimens were preserved in 70% ethanol solution and examined with a Leica DM LS2 trinocular microscope and a Leica MZ 12.5 trinocular stereomicroscope. All of the specimens were preserved in the Zoological Museum of Anadolu University (AUZM), Eskişehir. The specimens were identified according to the keys and descriptions from the works of Bauernfeind (

Although studies on the order Ephemeroptera are at a sufficient level in many of the provinces in the Aegean region, there is no comprehensive study on the provinces of Aydın and Denizli. Until now, four species from Aydın and two species from Denizli province have been reported from the Ephemeroptera order. The research being done in these provinces, which are the study regions, aims to fill in the gaps in the Aegean Area and provide more precise information about the national Ephemeroptera fauna. With the aid of sieves and water scoops, nymphs were gathered from the habitats of each locality's various biological characteristics during the field experiments, which were conducted in 25 different locations throughout two provinces. Thirteen species were recognized after examination of the 1177 collected specimens. In the study, five of the species identified from Aydın province and 12 of the species identified from Denizli province were recorded for the first time from the related provinces within the research area.