This study is aimed at comparing ostracod assemblages during the Eocene-Oligocene transition that were identified in the Paleogene succession at twenty-two previously studied locations across the seven geographical regions of Turkey. The formations deposited in the Eocene-Oligocene and the paleogeographic distribution and paleoenvironmental properties of the ostracods that were identified in these formations were investigated in the following different geographical regions of Turkey: the Aegean (Denizli), the Eastern Mediterranean (Mut, Karsant Baseni), Southeastern Anatolia (Adyaman), Eastern Anatolia (Malatya-Darende, Elaz), Central Anatolia (Karaman, Ulukla, Sivas, Ankara), Northern Anatolia (Bat Pontidler/Kastamonu) and Northwestern Anatolia (stanbul, Catalca/GD Trakya, Krklareli, Tekirda). The ostracod biozones in the acme-zone category were studied in these geographical regions. The zones were identified and chosen by successions or rock assemblages. The study revealed that the Eocene-Oligocene ostracod species demonstrated lacustrine, lagoonal, lagoon-littoral, epineritic, epineritic-infraneritic and infraneritic-bathyal environmental conditions. It was observed that, especially in the Aegean, Northwestern Anatolia and Southern Anatolia/Eastern Mediterranean regions, marl and silty sandstone in the upper and bottom levels of the Oligocene, in which coal settlement is observed, were usually deposited in lagoons containing ostracods. Furthermore, in the studied regions the co-occurrence of ostracod species living in maritime conditions with ostracod species living in lagoonal conditions can be explained by sea-related excessive shoaling in the basins. In Turkey, judging from environmental properties of the ostracod species, the transgression of the sea declined from northeast to northwest during the Eocene-Oligocene.