  We report a measurement of the $B^0_{s}$ lifetime in the semileptonic decay
channel $B^0_{s}\to D^-_s \mu^{+}\nu X$ (and its charge conjugate), using
approximately 0.4 fb$^{-1}$ of data collected with the D0 detector during 2002
-- 2004. We have reconstructed 5176 $D^-_s \mu^{+}$ signal events, where the
$D_s^-$ is identified via the decay $D_s^-\to \phi\pi^-$, followed by $\phi\to
K^+ K^-$. Using these events, we have measured the $B^0_s$ lifetime to be
$\tau(B^0_{s}) = 1.398 \pm 0.044$ $({stat}) ^{+0.028}_{-0.025}$ $({syst})
{ps}$. This is the most precise measurement of the $B_s^0$ lifetime to date.
