We present a measurement of the tt-bar differential cross
section with respect to the tt-bar invariant mass, d sigma/dM_tt-bar in
p anti-p collisions at s**(1/2) ñ.96 TeV using an integrated luminosity
of 2.7 fb-1 collected by the CDF II experiment. The tt-bar invariant
mass spectrum is sensitive to a variety of exotic particles decaying
into tt-bar pairs. The result is consistent with the standard model
expectation, as modeled by PYTHIA with CTEQ5L parton distribution
functions.