  We exploit the detection of three distinct stellar subpopulations in the red
giant branch of the Fornax dwarf Spheroidal to probe its density distribution.
This allows us to resolve directly the evolution with radius of the dark matter
mass profile. We find that a cored dark matter halo provides a perfect fit to
the data, being consistent with all three stellar populations well within
1-sigma, and for the first time we are able to put constraints on the core size
of such a halo. With respect to previous work, we do not strengthen the
statistical exclusion of a dark matter cusp in Fornax, but we find that
Navarro-Frenk-White haloes would be required to have unrealistically large
scale radii in order to be compatible with the data, hence low values of the
concentration parameter. We are then forced to conclude that the Fornax dwarf
Spheroidal sits within a dark matter halo having a constant density core, with
a core size of between 0.6 and 1.8 kpc.
