A supported titanium catalyst, adaptable for use in the stereoregular polymerization of .alpha.-olefins, is produced by treating a supported titanium-containing solid, obtained by cogrinding a magnesium halide, tetravalent titanium halide and organic acid ester, with a hydrocarbon, organo halogen compound or mixture thereof with heating. The resulting supported titanium catalyst, when employed as a titanium component with an organo aluminum catalyst component for the catalyst system in a process for stereoregular polymerization of .alpha.-olefins, produces high polymerization activity and unexpectedly high stereoregular polymer yielding ratios.