  We present a measurement of the B^0-B^0bar mixing parameter Delta m_d using
neutral B meson pairs in a 29.1 fb^{-1} data sample collected at the
Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy
e^+e^- collider. We exclusively reconstruct one neutral B meson in the
semileptonic B^0 \to D^{*-}\ell^+\nu decay mode and identify the flavor of the
accompanying B meson from its decay products. From the distribution of the time
intervals between the two flavor-tagged B meson decay points, we obtain Delta
m_d = (0.494 +- 0.012 +- 0.015) ps^{-1}, where the first error is statistical
and the second error is systematic.
