Abstract: Gas-phase polymerization of an .alpha.-olefin, especially propylene, in a stirred or fluidized bed of substantially uniform and approximately spherical carrier particles of an .alpha.-olefin polymer of diameter 100-500 .mu.m, using a catalyst comprising an organo-metallic compound and a solid transition metal compound which has a particle size of less than 5 .mu.m, produces a propylene polymer having unexpectedly good powder flow characteristics. The transition metal compound is conveniently a titanium trichloride-containing material which has been treated with a dialkyl ether solution.