The time dependence of B meson oscillations is studied using hadronic
Z(0) decays with identified leptons in both thrust hemispheres. Decay
times are reconstructed for each of the semileptonic B decays by formi
ng vertices which include the lepton and by estimating the B meson mom
entum. The mass difference of the two mass eigenstates in the B-d(0) s
ystem, Delta m(d), is measured to be 0.462 -0.040/+0.053 +0.052/0.035
ps(-1), where the first error is statistical and the second systematic
. For the B-S(0) system, a lower limit of Delta m(s) > 2.2 ps(-1) at 9
5% C.L. is derived.