  We study particle decay in de Sitter space-time as given by first order
perturbation theory in an interacting quantum field theory. We show that for
fields with masses above a critical mass $m_c$ there is no such thing as
particle stability, so that decays forbidden in flat space-time do occur there.
The lifetime of such a particle also turns out to be independent of its
velocity when that lifetime is comparable with de Sitter radius. Particles with
lower mass are even stranger: The masses of their decay products must obey
quantification rules, and their lifetime is zero.
