The mechanical properties of preconsolidated clays are studied at high pressures under undrained, triaxial conditions. At confining pressures equivalent to those at midcrust, different calys have the following common characteristics which differ significantly from those at low pressures: (1) the clays possess significant strength of several hundred bars (10 bars=1 MPa); (2) the constitutive relations are marked by ductile yielding and strain hardening, followed by a broad peak strength and a gradual decrease in strength at greater deformation; and (3) the change in volume during shearing is small. On the other hand, the behavior of montmorillonite differs from that of the other clays (illite, chlorite, and kaolinite) in the following ways: (1) its peak strength at a given confining pressure is about half of the strengths of the other clays; (2) its peak strength occurs at a shortening about 10%, while for the illite and chlorite the peak strength occurs at a shortening of 20--25% (for kaolinite, strain hardening continues even at 30% shortening); and (3) the fracture surfaces of some montmorillonite samples deformed at relatively low confining pressures show features resembling those in natural clayey fault gouge, whereas at higher confining pressures, montmorillonite and the other clays remain unfractured at shortening up to 30--40%.