Process for preparing copolymers of propylene with other 1-alkenes

A process for preparing a copolymer of propylene with another 1-alkene in an agitated fixed bed, which comprises gas-phase polymerizing a mixture of propylene and another 1-alkene in the absence of a liquid reaction medium under a preset pressure of from 15 to 30 bar, at from 65.degree. to 90.degree. C. and with an average holdup time of the polymer of from 1 to 5 hours, with the aid of a Ziegler-Natta catalyst system which contains an aluminum component and an organosilicon compound in addition to a titanium-containing solid component applied to a very finely divided carrier selected from the group consisting of silicas, aluminas and aluminum silicates of the formula SiO.sub.2. aAl.sub.2 O.sub.3, where a is from 0.01 to 2, wherein the temperature is chosen so that, under the preset pressure, no condensation of the monomer mixture takes place in the gas space and, wherein the ratio of the partial pressures of propylene and the other 1-alkene is adjusted to from 5:1 to 100:1.