The results of 408 Mcts linear polarization observations of the southern sky using the 210 ft steerable reflector at Parkes are presented. Combination of this survey with the northern sky survey of the Leiden group shows that almost all of the. polarization at this frequency lies in a band about 60� wide, which contains the great circle that passes through the galactic poles and intersects the plane at lil = 340� and 160�. This large-scale distribution of linear polarization at 408 Mcts may be explained on the basis of synchrotron radiation theory if the Sun lies almost at the centre of a spiral arm that has a magnetic field directed along it towards III = 70� and 250�. The concentrations of relativistic electrons may be confined to regions of higher than average magnetic field strength (5 X 10-5 G) elongated in the direction of the arm. The observations show that the magnetic fields in these "synchrotron" regions are very ordered. The distance to the polarized regions may be about 150 pc. At high latitudes, close correlation is observed between features of the distribution of background polarization and Faraday rotation of extragalactic sources.