Deep-water Middle Triassic succession in the southern Mt. Žumberak, deposited in a basin created because of extension tectonics, contains deposits which yielded remains of Middle Triassic ammonoids. Most of the fauna is stored in the collection of the Croatian Natural History Museum, while some specimens are new and therefore still not catalogued. Fossils determinable to the level of species or genus are hereby described in detail, discussed, and placed in palaeobiogeographic and biostratigraphic contexts. Likewise, basic geologic make-up of the terrain in question, lithological characteristics of the fossil-bearing rocks (limestone and calcareous-tuffaceous sandstone), and microfacies characteristics of limestone that made up the stone core of one of the described ammonoids, are briefly described. Using described fossils, as well as the ammonoid and bivalve fauna of Gregurić Breg, a relatively precise biostratigraphic analysis was performed. Results, together with older data based on microfossil assemblages, indicate that the deep-water succession of the Žumberak-Samoborsko gorje range has a stratigraphic range that spans the interval from the lower or middle Illyrian substage of the Anisian (trinodosus or reitzi ammonoid zones), to the uppermost Ladinian stage (neumayri or regoledanus ammonite zones).