During the Miocene, Paratethys Sea flooded a large part of present day northern Croatia. Because of the different composition of the rock substrate and expressed paleorelief, the transgressive deposits differ by age, lithological and paleontological characteristics. In the Medvednica area, Middle Miocene marine environments can be divided into three groups (“Doljanski”, “Čučerski” and “Zelinski” development), based on paleoecology and fossil fauna. Based on mollusks and associated fossil fauna, each development can be defined and clarified within by facies. Carried out research has supplemented existing collections of the Croatian Natural History Museum, which contain Badenian marine mollusks of Medvednica Mt., with new finds of bivalves and gastropods from all three developments. As part of this research, revision of these museum collections was carried out. Specimens of Pectinidae family (bivalves) have been scientifically revised, as well as new findings of planktic gastropod - pteropods from “Čučerje” and “Zelina” development. Along with them, rare and specialized bivalve fauna was recorded in “Čučerje” development, pointing to specific paleoecological conditions in certain Badenian developments. Findings of planktic gastropods indicate high sea levels, and allow reconstruction of migratory pathways during transgressive-regressive cycles in the Badenian. Based on the new findings and synthesis of present knowledge, an overview of transgressive-regressive cycles during the Badenian on today's Medvednica area was made. Deposits of transgressive-regressive cycle TB 2.4 (NN5 zone) are recorded in “Čučerje” and “Zelina” development and TB 2.5 (NN6 zone) in all three developments. Possibility of marine deposition in the oldest cycle [(?) TB 2.3 (NN4 zone)] is assumed in the former deepest parts of paleorelief in “Čučerje” development. Possible migratory path of mollusks in the Early Badenian was supposed “Transtethyan Corridor” in the west, which was probably opened until the Late Badenian, when existed also connection between Central and Eastern Paratethys.