We have measured the distribution of polarized emission from the radio galaxies NGC315, NGC6251, DA240, 3C236 and 3C326 at 327 MHz (92 cm wavelength) with the Westerbork Synthesis Radio Telescope (WSRT). These radio galaxies were all selected for observation because they exhibited degrees of polarization as high as 60 or 70 percent at 610 MHz (49 cm wavelength) and because they all have angular sizes greater than about 20 arcmin. Therefore they are still well-resolved by the WSRT at 92 cm wavelength. All sources in the sample exhibit significant linear polarization at 92 cm wavelength. Some depolarization does occur in NGC315 and 3C326 but parts of 3C236, DA240 and NGC6251 appear not to depolarize to 92 cm. Assuming an ordered, equipartition magnetic field, we find that the density of thermal depolarizing material in these radio galaxies is very low, typically about 1 × 10−5 cm−3 or less. Therefore these radio galaxies may represent distributions of pure magnetic fields and relativistic particles on scales of several hundred kiloparsecs or more.