This study of ostracods from 26 samples from the Aalenian and Lower Bajocian of Geisingen (Baden-Württemberg, southern Germany) has yielded five faunal assemblages. The assemblages from the Lower Aalenian are dominated by small-sized ostracod species and are in general less diverse than those assemblages recorded from the Middle and Upper Aalenian. First order microfaunal changes occur at the base of the Lower Aalenian “Comptum” Subzone, at the Bradfordensis/Gigantea Subzonal boundary and at the Bradfordensis/Concavum Zonal boundary. A second order microfaunal change is also noted at the Aalenian/Bajocian boundary (Concavum/Discites Zones). The following new species are described: Polycope circulosa, Cardobairdia tesakovae, Cytheropterina alacostata, Cytheropterina bicuneata, Infracytheropteron bisulcatum, Procytheropteron catena, Aphelocythere dilgeri, Praeschuleridea concentrica, Pleurocythere ohmerti, Progonocythere scutula, and Kinkelinella (K.) geisingensis. 13 presumably new species of Cytherurids, 1 “Monoceratina” sp. and 1 Kinkelinella sp. as well as 6 specimens ‘incertae sedis’are figured, but left in open nomenclature. The holotype and a paratype of Polycope pelta Fischer, 1961 are figured for the first time in SEM photographs.