An improved high activity olefin polymerization catalyst catalyzes the production of polymeric lower .alpha.-olefin having good properties and a relatively narrow particle size distribution. The catalyst is produced from an organoaluminum cocatalyst, a selectivity control agent and a novel olefin polymerization procatalyst which is prepared by contacting a tetravalent titanium halide, a halohydrocarbon, an electron donor and the solid procatalyst precursor obtained by heating an adduct of a carbonated magnesium ethoxide and a phenolic compound of enhanced acidity.