In the present study the ostracods from the Lower Badenian (Middle Miocene) Grund Formation of the Molasse Basin in Lower Austria are investigated. Most of the material comes from the type area of Grund. A total of 61 species could be differentiated within the Grund Formation. The following taxa dominate the faunal spectrum: Hemicytheridae (Aurila, Graptocythere, Grinioneis, Nonurocythereis, Pokornyella, Senesia, Tenedocythere, Urocythereis), Cyamocytheridea derii (Zalanyi, 1913), Cytheridea acuminata Bosquet, 1852 and Cytheridea paracuminata Kollmann, 1960. The ostracod assemblage reflects a shallow warm water environment with a water depth probably not exceeding 100 m. A part of the material is assumed to be re-deposited. Deep water elements are only represented by single specimens of Krithe, Henryhowella and Pseudocytherura. The stratigraphic classification of an Early Badenian age for the Grund Formation by means of Foraminifera can be supported mainly by the occurrence of Acanthocythereis hystrix (Reuss, 1850). Many species are also known either from Karpatian or from younger Badenian sequences in Austria. Furthermore a comparison between the ostracod contents of the formations of the former so-called Grund Beds is given (Karpatian: Laa and Korneuburg Formations; Lower Badenian: Grund and Gaindorf Formations). The occurrence of Loxoconcha vaisonna Carbonnel, 1969, Aurila larieyensis Moyes, 1965, Callistocythere karpatiensis Zorn, 1998 and Neocyprideis aff. corbleuensis Ducasse, 1995 in the Karpatian formations is most useful to separate them from the Badenian formations with the occurrence of Acanthocythereis hystrix (Reuss, 1850) and several species of Hemicytheridae.