Abstract The shielding properties of cylindrical pots of YBa 2Cu 3O 7− x (YBCO) prepared by isostatic compression were measured at 77 K for d.c. magnetic fields as well as a.c. magnetic fields in the range of frequencies from 100 Hz to 5 KHz. The critical penetration field ( H ∗), defined as the value of the applied magnetic field, at which a detectable field was observed inside the cylinder was 16 Gauss for isostatically compressed cylinders. The H ∗ values decreased as a function of frequency of the applied field was increased. The waveform of the field inside the cylinder for a sinusoidal applied field was highly distorted and showed the presence of higher harmonics with appreciale amplitude. The addition of 10% Ag seems to improve shielding at higher fields. The results are compared with that of a shield prepared by uniaxial compression.