  We study the proton flux expected from sources of ultra high energy cosmic
rays (UHECR) in the presence of regular extragalactic magnetic fields. It is
assumed that a local source of ultra-high energy protons and the magnetic field
are all in a wall of matter concentration with dimensions characteristic of the
supergalactic plane. For a single source, the observed proton flux and the
local cosmic ray energy spectrum depend strongly on the strength of the field,
the position of the observer, and the orientation of the field relative to the
observer's line of sight. Regular fields also affect protons emitted by sources
outside the local magnetic fields structure. We discuss the possibility that
such effects could contribute to an explanation of the excess of UHECR above
$5.10^{19}$ eV, and the possibility that sources of such particles may be
missed if such magnetic fields are not taken into account.
