  We present a set of seventeen Gamma-Ray Bursts (GRBs) with known redshifts
and X-ray afterglow emission. We apply cosmological corrections in order to
compare their fluxes normalized at a redshift of 1. Two classes of GRB can be
defined using their X-ray afterglow light curves. We show that the brightest
afterglows seem to decay faster than the dimer ones. We also point out
evidences for a possible flux limit of the X-ray afterglow depending on the
time elapsed since the burst. We try to interpret these observations in the
framework of the canonical fireball model of GRB afterglow emission.
