Carbonate concretions found within the Krosno shales (Polish Outer Carpathians) have formerly been interpreted as limestone exotics. Both the concretions and the host shales yield well preserved organic-walled dinoflagellate cysts. The dinoflagellate cyst assemblages provide valuable age-diagnostic information: they indicate a mid-Oligocene age and prove the concretionary origin of the carbonates. Detailed analysis of relative abundance, biodiversity and paleoecology of the dinoflagellates from concretions provides additional information on the sedimentary environment and the model of concretion formation.