ABSTRACT: In the “Hallstatt Mélange” in northwest Bad Mitterndorf, a slide of earliest late Pliensbachian marly radiolarites occurs in an upper-Middle to lower-Upper Jurassic succession. The succession consists of radiolarites, cherty limestones and marls, dated by radiolarians as upper-Middle Jurassic (Callovian) and different slides of Triassic to early Jurassic age derived from the Hallstatt facies zone. The facies and lithology (mainly cherty sediments) of the Pliensbachian slide are nearly identical to those of the succession, but it belongs to the Lower Jurassic Dürrnberg Formation of the outer-shelf area of the Northern Calcareous Alps (Hallstatt Zone), which was paleogeographically situated in the north-western rim of the Neotethys Ocean. The radiolarian fauna has low diversity but its good preservation allows an accurate age determination. The dating of the cherty slide as earliest late Pliensbachian is of great importance because: 1) it is the fi rst record of sediments younger than Sinemurian in the Hallstatt Zone of the Northern Calcareous Alps; 2) it is the fi rst record of Pliensbachian radiolarians in the European Alpine area; 3) it confi rms that the northwestern passive margin of the Neotethys Ocean persisted in this region at least until the Pliensbachian. Our paleontological and stratigraphic data prove that the closure of the Neotethys Ocean in this region is younger than earliest late Pliensbachian, but older than Callovian. The complete radiolarian fauna is illustrated and described. Fourteen taxa are identifi ed to species level, from which one new genus Paradroltus and two new species Paradroltus mitterndorfensis and Lantus sutnal are formally described.