ABSTRACT: Well-preserved Lower Jurassic (Hettangian-Sinemurian) radiolarian fossils were obtained from black chert and shale of the Wakai complex in the Ikuno area, Tamba Terrane, Southwest Japan. These faunas are dominated by the genera Natoba, Canoptum, Ovumella, and Droltus. We defi ne a Natoba-rich Assemblage as one containing >10% Natoba in the nassellarian assemblage. This assemblage is also characterized by relatively high abundance of Canoptum and Ovumella (combined abundance >15% nassellarian assemblage) and a low nassellarian/spumellarian ratio (<0.2). Based on data from Lower Jurassic radiolarian faunas from Japan, the Natoba-rich Assemblage is very similar to the Canoptum Assemblage. These two assemblages both contain common Natoba and occur exclusively in fi ne-grained siliciclastic rocks and black cherts. Considering the global distribution of Natoba, the Natoba-rich Assemblage appears to be restricted to oceanic regions peripheral to the western Panthalassa and Tethys.