The present invention concerns prepolymerized catalyst composition for polymerization of .alpha.-olefins, a process for the preparation thereof and a process for polymerization of .alpha.-olefins. According to the polymerization process a procatalyst composition containing a transition metal is prepolymerized with a monomer in order to produce a prepolymerized Ziegler-Natta type catalyst composition and said catalyst composition is contacted with an alpha-olefin monomer, in particular with ethylene or propylene, for preparing a polymer. According to the invention the procatalyst composition is preferably prepolymerized in a medium which is inert to the catalyst components, the viscosity of the medium being so high that the catalyst does not substantially settle in this medium. Because the catalyst composition according to the invention does not have to be dried before being fed into the medium, its activity is high before and during polymerization.