  We study decays of superheavy particles $X$ into leptons. We show that they
initiate cascades similar to QCD parton jets, if $m_X\gsim 10^6$ GeV.
Electroweak cascading is studied and the energy spectra of the produced leptons
are calculated in the framework of a broken SU(2) model of weak interactions.
As application, important for the Z-burst model for ultrahigh energy cosmic
rays, we consider decays of superheavy particles coupled on tree-level only to
neutrinos and derive stringent limit for these decays from the observed diffuse
extragalactic $\gamma$-ray flux.
