A highly effective catalyst system for the polymerisation of 1-olefins comprises an aluminoxane and a metallocene which is a compound of the formula I <IMAGE> in which R<1> and R<2> are hydrogen atoms, halogen atoms or hydrocarbon radicals, and R<3> and R<4> are monocyclic or polycyclic hydrocarbon radicals which can form a sandwich structure together with the central atom and which are connected to one another through a single- or multimembered, heteroatom-containing bridge -R<6>m-R<5>-R<7>n-. The catalyst system gives polymers of very high molecular weight.
