Acciarri, M (reprint author), IST NAZL FIS NUCL, VIA CELORIA 16, I-20133 MILAN, ITALY.
Time-dependent B-d(0)-(B) over bar(d)(0) mixing is studied using 1.5 million hadronic Z decays collected by L3. Semileptonic B decays are selected by requiring at least one reconstructed lepton in both thrust hemispheres. Charge correlations between the tagged leptons are studied as a function of proper time. The proper time of the b-hadron decay is measured by reconstructing the production and decay vertices using a silicon microvertex detector: The measured B-d(0) meson oscillation frequency corresponds to a mass difference Delta m(d) between the two B-d(0) mass eigenstates of Delta m(d) = (0.496(-0.051)(+0.053) (stat) +/- 0.043 (syst)) ps(-1).