We present a measurement of the mass difference Delta m_d mass
eigenstates.  We use a flavor tagging method based on the lepton charge, in a
sample of events with two muons at low transverse momentum.  The sample
corresponds to an integrated luminosity of 90 pb-1 collected by the Collider
Detector at Fermilab.  The result obtained is Delta m_d = 0.503 +/-
0.064(stat.) +/- 0.071(syst.) ps-1.