The first contribution dealing with the study of Middle Jurassic radiolarians in the Western Carpathians presents the composition of radiolarian associations in the Upper Posidonia beds of the Pieniny succession (s. 1.) in the Klippen Belt at Trstena in Orava. The assemblages have been assigned to the Lower to Middle Callovian. For comparison, the composition of an assemblage from the overlying radiolarite horizon of Upper Callovian to Upper Oxfordian age has been included.