A prepolymerized solid catalyst for gaseous phase or slurry polymerization of alpha olefins is disclosed. The catalyst comprises a porous carrier having a particle size of 20 to 200 microns onto which has been deposited an aluminoxane and a transition metal compound having at least one cycloalkadienyl group, the transition metal being from group 4 of the Periodic Table. The transition metal is present in an amount of 0.003 to 3 mmol/gm of carrier, and the aluminum to transition metal atomic ratio is from 15 to 1000. This catalyst is then prepolymerized with 0.2 to 30 gm of an alpha olefin per gram of catalyst to form the final prepolymerized solid catalyst having an intrinsic viscosity of 0.2 to 20 dl/gm as measured in decalin at 135 deg C.