Propylene-ethylene copolymers are prepared in an agitated bed of solids by first polymerizing propylene from the gas phase in a first polymerization zone at 20-40 bar and 60.degree.-90.degree. C. by addition of a Ziegler-Natta catalyst system, and copolymerizing in a second polymerization zone at 7-35 bar and 40.degree.-70.degree. C. the finely divided polymer from the first polymerization zone with a mixture of olefins a process which comprises copolymerizing in the second polymerization zone with a mixture of propylene, ethylene and a further .alpha.-olefin of from 4 to 10 carbon atoms, setting the ratio of the partial pressures between propylene and ethylene within the range from 100:20 to 100:120 and that of the partial pressures between propylene and the further .alpha.-olefin within the range from 100:1 to 100:20, setting the pressure in the second polymerization zone at at least 5 bar below the pressure in the first polymerization zone, and determining the weight ratio between the monomers reacted in the first polymerization zone and those reacted in the second polymerization zone in such a way that said weight ratio is within the range from 100:10 to 100:50.