During the period of ca. 20 Ma (Middle Eocene–Chattian) the Leluchów Succession of the Magura Basin passed through drastic changes of sedimentary condition and paleobathymetry from well oxygened red shales with Reticulofragmium amplectens, deposited beneath CCD, red Globigerina oozes, to oxygen depleted organic-rich menilite-type shales and finally to flysch deposition of open marine conditions. The biostratigraphic and lithostratigraphic scheme is well established with the Leluchów Marl Member — Zones NP19–20 to NP22 (Late Eocene–Early Oligocene), Smereczek Shale Member, Zone NP23 (Early Oligocene) and the Malcov Formation s.s., Zone NP24 (Early–Late Oligocene). The aim of the paper is to present the quantitative analyses as the basis for paleoecological changes in the Magura Basin during the Late Eocene–Late Oligocene period. The changes manifest themselves through a decrease in the water temperature and progressing eutrophication. Species typical of brackish water conditions and restricted to the Paratethys region were identified from the NP23 Zone.