We present a measurement of the mass difference Delta m(d) of the two B-d(0
) mass eigenstates. We use a flavor tagging method based on the lepton char
ge, in a sample of events with two muons at low transverse momentum. The sa
mple 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). [S0556-2821(99)50215-4].