The aim of this study is to determine of ostracode fauna, to do their chronostratigraphical and environmental evolution of Bakırköy formation Which crops out Bahçelievler, Cennet Mahallesi and Avcılar (İs-tanbul-Küçükmece) according to new observations. Argillaceous limestone, claystone and fossiliferous limestone levels of Bakırköy formation have been obser¬ved which concordantly overly Güngören formation that named as İstanbul green clay in measured section of Avcılar, Küçükçekmece. Bakırköy formation also is represented by limestones and argillaceous limestones which those overlying claystones at Cennet Mahallesi which is located in the eastern part of investigated area. Bakırköy formation consists of the alternation of fossiliferous, argilleous limestone, claystone and sandy limestone at the uppermost of Bahçelievler measured section. 43 samples have been collected and studied from this unit with this research As a result of paleontological studies, two different ostracode associations have been described according to the percentage of salinity. Leptocythere nodigera Pokorny, Leptocythere castanae (Sars), Loxoconcha rhombovalis Pokorny, Loxoconcha granifera (Reuss), Xestoleberis reymenti Ruggieri, Xestoleberis ventricosa Mueller, Cyprideis heterostigma sublittoralis Pokorny, Cyprideis sublittoralis Pokorny, Cyprideis torosa (Jones), Cyprideis trituberculata (Krstic) are brakisch ostracodes and Heterocypris salina salina (Brady), llyocypris bradyi (Sars), Cyclocypris sp.,Candona Candida (O.F.Muller), Candona neglecta Sars, Candona (Candona) parallela pannonica Zalanyi, Candona decimal Freels, Darwinula cylindrica Straub, Darwinula stevensoni (Brady ve Robertson) are from the freshwater association. According to this microfauna content, Bakırköy formation has been deposited in a lagoon which has mostly continental in put. Finally, Bakırköy formation which is dated Sarmatian in the previous studies, has been evaluated Pontian in age according to the ostracode fauna and by the correlation of Messinian-Pontian aged green clays of Istanbul from the eastern part of the study area.