We report a measurement of the diffractive structure function
of the antiproton for dijet events produced in association with a
leading antiproton in p anti-p collisions at s**(1/2) = 1800 GeV. Comparing
our results with expectations based on parton densities extracted by the
H1 Collaboration from diffractive deep inelastic scattering at the DESY
ep collider HERA, we find a severe breakdown of QCD factorization.