  If the energy density of the Universe before nucleosynthesis is dominated by
a scalar field $\phi$ that decays and reheats the plasma to a low reheating
temperature $T_{RH}$, neutralinos may be warm dark matter particles. We study
this possibility and derive the conditions on the production mechanism and on
the supersymmetric spectrum for which it is viable. Large values of the $\mu$
parameter and of the slepton masses are characteristic features of these
models. We compute the expected direct detection cross sections and point out
that Split-SUSY provides a natural framework for neutralino warm dark matter
