An early Middle Jurassic (Aalenian) radiolarian fauna is reported from the Xialu chert in the Yarlung Zangbo Suture Zone. This is the oldest known fossil record from the chert. The fauna comes from a chert sample which contains no terrigenous elements other than clay minerals. This indicates that the Ceno-Tethys was already a rather wide ocean with a pelagic environment far from continents by that time. The rifting and separation of the Lhasa Block from the northern Gondwana margin must have occurred well before early Middle Jurassic time.