Polymerization of ethylene catalyzed by a mixture of a Ziegler-Natta (Z-N) and a metallocene system cocatalyzed with methylaluminoxane (MAO) were carried out in a single reactor. The Z-N catalyst can be activated in the presence of MAO, without any addition of a conventional Z-N activator. The activities of the mixture Z-N with Cp2MCl2 (M = Ti,Zr,Hf) (1,2,3) or CpZrCl3 (4) were lower than the obtained with the mixture Z-N and en(Ind)(2)ZrCl2 (5). The weight-average molecular weight (Mw) of the polyethylene (PE) obtained with the mixture Z-N with Cp2MCl2 (M = Ti,Zr,Hf) or en(Ind)(2)ZrCl2 was lower than the PE obtained with Z-N and CpZrCl3. For the binary mixture of Z-N with CpZrCl3 (4), the PE obtained showed a bimodal molecular weight distribution (MWD) for an appropriate composition of the mixed catalyst in the absence of hydrogen in the reaction medium.