Conventional Ziegler-Natta catalysts, based on TiCl4 supported on MgCl2, were modified by substituting ct part of the chlorides by cyclopentadienyl (and derivatives) ligands. Although these catalysts are very active (activities up to 10(5) g PE/g catalyst/h) they exhibit a conventional Ziegler-Natta behavior (methylaluminoxane is not necessary, polyethylene produced with a rather broad molar weight distribution, low sensitivity to hydrogen). It was attributed to cluster effects : an increase of the number of conventional TiCl4 active sites by adding cyclopentadienyl ligands on titanium neighbors.