  The supersymmetric generalization of the limiting and Gaussian QCD spectra is
obtained. These spectra are valid for $x \ll 1$, when the main contribution to
the parton cascade is given by gluons and gluinos. The derived spectra are
applied to decaying superheavy particles with masses up to the GUT scale. These
particles can be relics from the Big Bang or produced by topological defects
and could give rise to the observed ultrahigh energy cosmic rays. General
formulae for the fluxes of protons, photons and neutrinos due to decays of
superheavy particles are obtained.
