In this study, the gastropod fauna of the sediments around the Beyşehir Lake basin were examined, and the paleoecological features of the old lake were revealed with the help of fauna. As a result of the examination of the samples taken from the sediments of the old Beyşehir Lake, it belongs to the Gastropoda class of the Mollusca branch; Theodoxus heldreichi Martens, Valvata pulchella Studer, Valvata beysehirensis Glöer&Girod, Valvata sp., Bithynia tentaculata Linnaeus, Bithynia leachii Linnaeus, Bithynia sp., Lmynaea stagnalis Linnaeus, Lmynaenicolastrig. auricularia Linnaeus, Radix ovata Linnaeus, Planorbis planorbis Linnaeus, Anisus vortex Linnaeus, Gyraulus parvus Say, Gyraulus sp., Planorbarius corneus Linnaeus, Planorbarius sp., Bathyomphalus contortus Linnaeus, 15 species have been defined. When the general paleoecological characteristics and numerical abundances of the identified species are interpreted, it is seen that Valvata pulchella and Bithynia leachii, which are the most numerous, are the typical dominant species of the Eski Beyşehir lake. The presence of Valvata pulchella, which lives in streams at the levels where Bithynia leachii is found, indicates that the lake is a freshwater lake fed by streams at these levels. The levels where the forms that feed on plants such as Planorbis planorbis, Anisus vortex, Gyraulus parvus and live in stagnant and shallow environments and swamps are abundant in number indicate a very shallow closed lake and temperate conditions. Valvata beysehirensis, which is the endemic species of the old Beyşehir lake and reflects the freshwater environment fed by rivers, has been observed in many levels of the basin. It has been observed that the V. beysehirensis species described by Girod (2013) and given a Middle Pleistocene age can also reach the lower and upper levels of the Quaternary with this study. This study was supported by Çanakkale Onsekiz Mart University Scientific Research Projects Coordination Unit. Project Number: FDK-2020-3340.