A study on a new generation of MgCl2-supported Ziegler-Natta catalysts, which contain dialkylpropane diethers as internal donors and have the property of producing highly active and stereospecific catalysts without any external donor, has been performed. The behavior of three different diethers (2,2-diisobutyl-1,3-dimethoxypropane, 2,2-dicyclopentyl-1,3-dimethoxypropane, and 2-ethyl-2-butyl-1,3-dimethoxypropane) as internal donors is compared with the behavior of the same diethers when they are used as external donors with a catalyst containing diisobutyl phthalate as internal donor. The results obtained show that the same isospecific centers are produced when the same diether donor is present as an internal donor or replaces diisobutyl phthalate during polymerization when it is added as an external donor. The sum of the present results and of the previously obtained findings on traditional supported Ziegler-Natta catalysts has enabled us to reach some conclusions on the role and reciprocal behavior of internal and external donors.