Abstract: There are disclosed highly active supported catalysts for the polymerization of olefins to polymers in the form of spheroidal particles which neither break nor cake when subjected to compression. The supported catalysts are prepared by mixingA. a catalyst-forming component which is an organometallic compound of a metal belonging to Group II or III of the Mendelyeev Periodic TableWithB. a supported catalyst-forming component which is the product consisting of a carrier comprising an anhydrous Mg halide and of a halogenated Ti compound chemically combined with the carrier or dispersed on the same, said product being in the form of spherical or spheroidal particles of a size comprised between 1 and 350 microns and having characteristics as set forth in detail hereinbelow.