Three manganese complexes, Mn(acac)(3) (acac = acetylacetonate), Cp2Mn (Cp = cyclopentadienyl), and Mn(salen)CI [salen = 1,2-cyclohexanediamino-NN'-bis(3,5-dit-butyl-salicylidene)], were used for ethylene and propylene polymerizations. These complexes, in combination with an alkylaluminum cocatalyst such as methylaluminoxane (MAO) or diethyl aluminum chloride (AlEt2Cl), could promote ethylene polymerizations that yielded extremely high molecular weight linear polymers, but were inactive for propylene polymerizations. The counterparts supported on MgCl2 showed activities even for propylene polymerizations and had remarkably enhanced activities for ethylene polymerizations. In the presence of an electron donor such as ethylbenzoate, the MgCl2-supported manganese-based catalysts yielded a highly isotactic polypropylene with a high molecular weight.