  The discrepancies in the results produced by the two most commonly used Monte
Carlo programs for simulation of propagation of ultra-high energy cosmic ray
photons in the presence of the geomagnetic field are presented. Although
photons have not yet been discovered in the cosmic ray flux at highest
energies, the capabilities of the present cosmic ray detectors make their
discovery possible, according to the predictions of conventional models, within
the next few years. It is therefore necessary to have a reliable and well
maintained software for relevant simulations. The results of this paper are
important for simulations of propagation of photons at energies above 10^19 eV.
Photons of such high energies might interact with the geomagnetic field giving
rise to a cascade of particles even above the atmosphere. This effect is called
a "preshower effect". The preshower effect is important for air shower
evolution and has to be accounted for in full Monte Carlo simulations of
propagation of highest energy cosmic-ray photons. In this paper we compare the
two most frequently used Monte Carlo codes for preshower simulations:
PRESHOWER, used as a stand-alone program or as a part of CORSIKA, and MaGICS,
used as a part of AIRES.
