We report a precise determination of the Z0-B̄0 mixing parameter Δmd based on the time evolution of same-sign and opposite-sign dilepton yields in Y(45) decays. Data were collected with the Belle detector at KEKB. Using data samples of 29.4 fb-1 recorded at the γ(4S) resonance and 3.0 fb-1 recorded at an energy 60 MeV below the resonance, we measure Δmd=[0.503±0.008(stat) ±0.010(syst)] ps-1. From the same analysis, we also measure the ratio of charged and neutral B meson production at the γ(4S), f +/f0= 1-01 ±0.03(stat)±0.09(syst), and CPT violation parameters in B0-B̄0 mixing, (cos θ)=0.00±0.12(stat) ±0.01(syst) and ℑ(cos θ)=0.03±0.01(stat)±0.03(syst). © 2003 The American Physical Society.