Abstract: Particulate polymers of ethylene are prepared by polymerization of ethylene or ethylene/.alpha. -monoalkene mixtures in a dry agitated bed (B) of the particulate polymer by means of a Ziegler/Natta catalyst system consisting of (1) a titanium-containing component and (2) an aluminum-containing component, the said components (1) and (2) being separately fed to the bed (B). Component (1) is a particulate reaction product (U) (of) (1.1) a substance of the formula TiCl.sub.3 . (AlCl.sub.3).sub.n where n is a number equal to from 0 to 0.34, and (1.2) a reaction product (R) of phosgene and a substnace (S) obtained from a substance of the formula Mg.sub.6 . Al.sub.2 . (OH).sub.16 . CO.sub.3 . (H.sub.2 O).sub.4 by calcination (heating), provided that (R) has a chlorine content of from 10 to 76% by weight and (U) has been obtained by milling (1.1) and (1.2) together.