We present a measurement of the tt differential cross section with respect to the tt invariant mass, dσ/dMtt, in pp collisions at s√=1.96  TeV using an integrated luminosity of 2.7  fb−1 collected by the CDF II experiment. The tt invariant mass spectrum is sensitive to a variety of exotic particles decaying into tt¯ pairs. The result is consistent with the standard model expectation, as modeled by PYTHIA with CTEQ5L parton distribution functions.