Stereospecific polymerization of 1-hexene under high pressures (up to 1,000 MPa = ca. 10,000 atm) using metallocene/methylaluminoxane (MAO) catalysts was investigated. Several C-2-symmetric ansa-metallocenes, their meso-isomers, and two C-s-symmetric ansa-metallocenes were employed as catalyst precursors. In the course of this study, novel C-2-symmetric germylene-bridged ansa-metallocenes, (rac-[Me2Ge(eta(5)-C5H-2,3,5-Me-3)(2)MCl2] (M = Zr, rac-4a; M = Hf, rac-4b), have been prepared. High pressures induced enhancement of the catalytic activity and the molecular weight of the polymers in most of the catalysts. The maximum of both the catalytic activity and the molecular weight of the polymers was mostly observed at 100-500 MPa in each catalyst, although the enhanced ratio was smaller than that observed for nonbridged metallocenes. Isospecificity of the C-2-symmetric ansa-metallocene catalysts was essentially maintained even under high pressure. Highly isotactic polyhexene ([mmmm] = 91.6%) with very high molecular weight (M-w = 2,360,000) was achieved by rac-4b under 250 MPa. High pressures slightly decreased syndiotacticity when the C-s-symmetric ansa-metallocene, isopropylidene(1-eta(5)-cyclopentadienyl)(9-eta(5)-fluorenyl)zirconium dichloride 5, was employed.