Catalyst compositions for homopolymerization and copolymerization of ethylene which comprise two transition metal compounds, one of them a cyclopentadienyl complex of a transition metal and another a non-metallocene derivative of a transition metal are described. The catalysts are activated by alkylalumoxanes that are soluble in non-aromatic hydrocarbons. Bimetallic catalysts of this invention are suitable for the manufacture of ethylene homopolymers and copolymers with broad bimodal molecular weight distributions. The alkyl alumoxanes have at least one [AR(R)--O--] repeating group in which R is an alkyl group of at least two carbon atoms.