  In theories with a gauge-mediated mechanism of supersymmetry breaking the
gravitino is likely to be the lightest superparticle and, hence, a candidate
for dark matter. We show that the decay of the next-to-lightest superparticle
into a gravitino can yield a non-thermal population of gravitinos which behave
as a hot dark matter component. Together with the warm component, which is
provided by the population of gravitinos of thermal origin, they can give rise
to viable schemes of mixed dark matter. This realization has some specific and
testable features both in particle physics and astrophysics. We outline under
which conditions the mechanism remains viable even when R parity is broken.
