Abstract: High yields of elastomeric copolymers of ethylene and higher alpha-olefins, or of ethylene, higher alpha-olefins and unsaturated monomers (polyenes) containing two or more double bonds are obtained by copolymerizing a mixture of the monomers in contact with a catalyst prepared by mixing a catalyst-forming component which is an organometallic compound of aluminum with a supported catalyst-forming component obtained by contacting a halogenated titanium compound with a carrier or support consisting of or comprising an anhydrous Mg or Mn halide partially or completely complexed with a Lewis base capable of forming a complex with it.The copolymers so obtained have valuable mechanical properties, in particular a relatively low value of permanent set and do not exhibit excessive residual deformation at break.