  We present a measurement of the mass difference m(D+_s) - m(D+), where both
the D+_s and D+ are reconstructed in the phi pi+ decay channel. This
measurement uses 11.6 pb-1 of data collected by CDF II using the new
displaced-track trigger. The mass difference is found to be: 99.41 +- 0.38
(stat) +- 0.21 (syst) MeV/c^2.
