  We study the Hamiltonian Mean Field (HMF) model, a system of $N$ fully
coupled particles, in the microcanonical ensemble. We use the previously
obtained free energy in the canonical ensemble to derive entropy as a function
of energy, using Legendre transform techniques. The temperature-energy relation
is found to coincide with the one obtained in the canonical ensemble and
includes a {\it metastable} branch which represents spatially homogeneous
states below the critical energy. "Water bag" states, with removed tails
momentum distribution, lying on this branch, are shown to relax to equilibrium
on a time which diverges linearly with $N$ in an energy region just below the
phase transition.
