  The oscillation frequency, $\Delta m_d$, of $B^0$ mesons is measured using
semileptonic decays with a $D^-$ or $D^{*-}$ meson in the final state, in a
data sample of $pp$ collisions collected by the LHCb detector corresponding to
an integrated luminosity of 3.0$\mbox{fb}^{-1}$. A combination of the two decay
modes gives $\Delta m_d = (505.0 \pm 2.1 \pm 1.0) \rm \,ns^{-1}$, where the
first uncertainty is statistical and the second is systematic. This is the most
precise single measurement of this parameter. It is compatible with the current
world average and has similar precision.
