We report a measurement of the B-0-(B) over bar (0) mixing parameter Deltam(d) based on a 29.1 fb(-1) sample of Y(4S) resonance decays collected by the Belle detector at the KEKB asymmetric e(+)e(-) collider. We use events with a partially reconstructed B-0((B) over bar (0)) --> D*(-/+)pi(+/-) candidate and where the flavor of the accompanying B meson is identified by the charge of the lepton from a B-0((B) over bar (0)) --> X(-/+)l(+/-)nu decay. The proper-time difference between the two B mesons is determined from the distance between the two decay vertices. From a simultaneous fit to the proper-time distributions for the same-flavor (B-0((B) over bar (0)), l(+/-)) and opposite-flavor (B-0((B) over bar (0)), l(-/+)) event samples, we measure the mass difference between the two mass eigenstates of the neutral B meson to be Deltam(d)=(0.509 +/- 0.017 (stat) +/- 0.020 (syst)) ps(-1).