ABSTRACT: The ostracod faunas of six middle to late Eocene sections in the Central Sakarya Region were studied with the aim to describe the palaeoenvironmental changes occurred in the area and the age of the Halidiye Formation. This study based on theOstracoda faunawas carried out for the first time in the region and a comparison between theOstracoda species identified in this study and the data obtained by other previous work carried out in central Europe and Turkey indicated a middle to late Eocene age for the formation. Within the Halidiye Formation, 35 species of Ostracoda belonging to 31 genera were observed, among which, the followings are the more relevant for chronostratigraphic purposes: Cytherella jonesiana (Bosquet), Bairdia cymbulaDeltel, B. crebraDeltel, Neocyprideis williamsoniana (Bosquet), Cuneocythere (Monsmirabilia) triebeli (Keij), Schuleridea perforata (Roemer), Hemicyprideis montosa (Jones ve Sherborn), Cladarocythere apostolescui (Margerie), Krithe parvula Deltel, K. angusta Deltel, K. caudata Van Den Bold, K. rutoti Keij, Trachyleberis aculeata modesta Van Hinte, Pterygocythereis aquitanica Ducasse, Nucleolina multicostata (Deltel), Leguminocythereis genappensis Keij, Grinioneis alata Ducasse, G. paijenborchiana Keij, Quadracythere persica Tambareau, Caudites orientalis Sönmez-Gökçen, Loxoconcha subovata (Muenster), Monoceratina striata Deltel, Uroleberis globosa Ducasse, Paracypris contracta (Jones), Cyprinotus sp., and Heterocypris sp. Based on the lithological properties and environmental indicators, the Halidiye Formation reflects characteristics of transitional lagoonal to deep marine settings. Among the Ostracoda identified hereby, Cyprinotus and Heterocypris indicate freshwater, Cladarocythere, Neocyprideis, and Loxoconcha indicate lagoonal, Leptocythere, Cytheromorpha, and Cyamocytheridea indicate lagoonallittoral, Hermanites, Quadracythere, and Uroleberis indicate epineritic, Monsmirabilia, Schuleridea, and Caudites indicate epineritic-infraneritic, Paracypris indicates infraneritic, Cytherella, Bairdia, and Cytheropteron indicate epineritic-bathyal, and, finally, Krithe and Macrocypris indicate infraneritic-bathyal environmental conditions.