  We show how to assign, on two intersecting null hypersurfaces, initial data
for the Einstein-Vlasov system in harmonic coordinates. As all the components
of the metric appear in each component of the stress-energy tensor, the
hierarchical method of Rendall can not apply strictly speaking. To overcome
this difficulty, an additional assumption have been imposed to the metric on
the initial hypersurfaces. Consequently, the distribution function is
constrained to satisfy some integral equations on the initial hypersurfaces.
