Process for the polymerization of olefins CH.sub.2.dbd.CHR, in which R is hydrogen or a hydrocarbon radical with 1 12 carbon atoms, carried out in the presence of a catalyst component (A) comprising Mg, Ti and halogen as essential elements and of a catalyst component (B) capable to produce, under the same polymerization conditions, a polymer with an average particle size lower than that obtainable with the said catalyst component A. The said process provides polymers with increased bulk density.