  A deep radio observation of the A2255 cluster of galaxies has been carried
out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the
excellent (u,v) coverage and sensitivity achieved by our observation, the low
brightness diffuse extended sources in the cluster (radio halo and relic) have
been imaged with unprecedented resolution and dynamic range. We find that the
radio halo has filamentary structures that are strongly polarized. The
fractional linear polarization reaches levels of $\simeq$ 20$-$40% and the
magnetic fields appear ordered on scales of $\sim$400 kpc. This is the first
successful attempt to detect polarized emission from a radio halo and provides
strong evidence that in this cluster the magnetic field is ordered on large
scales.
