  We report a measurement of the Lambda_b lifetime using a sample corresponding
to 1.3 fb$^{-1}$ of data collected by the D0 experiment in 2002--2006 during
Run II of the Fermilab Tevatron collider. The Lambda_b baryon is reconstructed
via the decay Lambda_b -> mu nu Lambda_c X. Using $4437 \pm 329$ signal
candidates, we measure the Lambda_b lifetime to be $\tau(Lambda_b)$ =
1.290^{+0.119}_{-0.110}(stat) ^{+0.087}_{-0.091} (syst) ps, which is among the
most precise measurements in semileptonic Lambda_b decays. This result is in
good agreement with the world average value.
