  CP violation in B0B0bar mixing is characterized by the value of the parameter
|q/p| being different from 1, and the Standard Model predicts this difference
to be smaller than 10^{-3}. We present a measurement of this parameter using a
partial reconstruction of one of the B mesons in the semileptonic channel D*-
l+ nu_l, where only the hard lepton and the soft pion from the D*- --> D0bar
pi- decay are reconstructed. The flavor of the other B is determined by means
of lepton tagging. The determination of |q/p| is then performed with a fit to
the proper time difference of the two B decays. We use a luminosity of 200.8
fb^-1, collected at the Y(4S) resonance by the BaBar detector at the PEP-II
asymmetrical-energy e+e- collider, in the period 1999-2004. We obtain the
preliminary result:
  |q/p| - 1 = (6.5 +/- 3.4(stat.) +/- 2.0(syst.)) x 10^{-3}
