A morphological investigation was carried out on different Ziegler-Natta catalysts during the early stages of propylene homo- and propylene-ethylene copolymerization. For similar polymerization conditions, but dependent on the nature of the catalysts, fragmentation occurs layer-by-layer or instantaneously into a large amount of small pieces. However, the incorporation of comonomer ethylene slows down the fragmentation progress. This is believed to be the result of the higher mobility of the just formed propylene-ethylene copolymer molecules at the active sites.