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The oscillation frequency Δmd of B0B̅0 mixing is measured using the partially reconstructed semileptonic decay B̅0→l- ν̅ D*+X. The data sample was collected with the CDF detector at the Fermilab Tevatron collider during 1992–1995 by triggering on the existence of two lepton candidates in an event, and corresponds to about 110 pb-1 of p̅p collisions at √s =1.8 TeV. We estimate the proper decay time of the B̅0 meson from the measured decay length and reconstructed momentum of the l-D*+ system. The charge of the lepton in the final state identifies the flavor of the B̅0 meson at its decay. The second lepton in the event is used to infer the flavor of the B̅0 meson at production. We measure the oscillation frequency to be Δmd=0.516 ±0.099-0.035+0.029ps-1, where the first uncertainty is statistical and the second is systematic.