  We report a precise determination of the B0-B0bar mixing parameter Delta-m_d
based on the time evolution of same-sign and opposite-sign dilepton yields in
Upsilon(4S) decays. Data were collected with the Belle detector at KEKB. Using
data samples of 29.4 1/fb recorded at the Upsilon(4S) resonance and 3.0 1/fb
recorded at an energy 60MeV below the resonance, we measure Delta m_d = (0.503
+- 0.008(stat) +- 0.010(sys)) 1/ps. From the same analysis, we also measure the
ratio of charged and neutral B meson production at the Upsilon(4S), f+/f0 =
1.01 +- 0.03(stat) +- 0.09(sys), and CPT violation parameters in B0-B0bar
mixing, Re(cos theta) = 0.00 +- 0.12(stat) +- 0.01(sys) and Im(cos theta) =
0.03 +- 0.01(stat) +- 0.03(sys).
