We report the stereospecific polymerization of methyl methacrylate (MMA) with single-component cationic zirconocene catalysts. The complexes Me(2)CCpIndZrMe(thf)+BPh4- (1) and Me2CCp2ZrMe(thf)+BPh4- (2) are active catalysts for the polymerization of MMA, in remarkable contrast to other cationic zirconocenes. While 1 yields highly isotactic poly(methyl methacrylate) (PMMA), 2 is syndiospecific at low temperatures. This is the first example for a rational control of PMMA microstructure via catalyst symmetry. Polymerization kinetics and stereospecificity control in MMA polymerization with 1 and 2 are discussed. On the basis of the experimental data, a possible polymerization mechanism is proposed.