This invention is for a metallocene catalyst component useful in preparing isotactic polyolefins, a catalyst system using this component and a process for polymerizing olefins using the catalyst component. The catalyst component consists of a bridged metallocene in which one of the cyclopentadienyl rings is substituted in a different manner from the other ring. One of the cyclopentadienyl ligands contains a bulky group in one and only one distal position and the other cyclopentadienyl ligand has bilateral or pseudobilateral symmetry. There is a steric difference between the two Cp rings.The catalyst component is a stereorigid metallocene described by the formula:R" (CpR.sub.n)(CpR'.sub.m)MeQ.sub.kwherein each Cp is a cyclopentadienyl or substituted cyclopentadienyl ring; each R.sub.n and R'.sub.m is a hydrocarbyl radical having 1-20 carbon atoms; R" is a structural bridge between the two Cp rings imparting stereorigidity to the Cp rings; Me is a transition metal; and each Q is a hydrocarbyl radical or is a halogen. R'.sub.m is selected so that (CpR'.sub.m) is a sterically different substituted cyclopentadienyl ring than (CpR'.sub.n). The use of a metallocene catalyst component with sterically different cyclopentadienyl rings in which one and only one of the distal positions of the cyclopentadienyl rings has a bulky group as a substituent produces isotactic polymer.