Abstract: The present invention relates to a process for the polymerization of alpha-olefins in the gas phase in the presence of a Ziegler-Natta catalyst system consisting of a catalyst comprising atoms of halogen, magnesium and a transition metal, and cocatalysts based on an organometallic compound of a metal of Groups II or III of the Periodic Table of Elements, this catalyst system being previously converted into a prepolymer or a catalyst supported on a granular substance. This process is characterized in that the polymerization in the gas phase is carried out by contacting one or more alpha-olefins, on the one hand with the prepolymer or the supported catalyst system comprising an organometallic compound (a) which is only slightly volatile as cocatalyst, and on the other hand another cocatalyst consisting of an organometallic compound (b) which is relatively volatile, in such a way that the total quantity of cocatalyst employed is relatively small.