A catalyst which exhibits superior control of product particle size in the insertion polymerization of alpha-olefins is described as transition metal compounds which associate with non-coordinating alumoxane anions which are dispersed throughout the interior of a silica support in the form of particles which are less than or equal to about 50 .ANG. (5 nanometers) in diameter. This catalyst is capable of gas phase (e.g. heterogeneous) polymerization of propylene into product granules with a high degree of control over granule particle size distribution and bulk density.