Catalyst components for the preparation of ethylene (co)polymers capable to produce polymers with high bulk density, to avoid the problems of formation of fines and that have a high activity are provided. Said catalyst components are characterized by comprising a non-stereospecific solid catalyst component, comprising a titanium compound and a magnesium dihalide, which is pre-polymerized with an alpha olefin CH.sub.2.dbd.CHR.sup.1 wherein R is a C1 C8 alkyl group, to such an extent that the amount of the .alpha.-olefin pre-polymer is up to 100 g per g of said solid catalyst component.