  The lifetimes of the B- and B0-bar mesons are measured using the partially
reconstructed semileptonic decays B-bar -> D l- nu-bar X, where D is either a
D0 or D*+ meson. The data were collected by the CDF detector at the Fermilab
Tevatron collider during 1992-95 and correspond to about 110 pb-1 of p-bar p
collisions at sqrt(s)=1.8 TeV. We measure decay lengths and extract the
lifetimes to be tau(B-) = 1.637 +/- 0.058 +0.045 -0.043 ps, tau(B0) = 1.474 +/-
0.039 +0.052 -0.051 ps, and the ratio of the lifetimes to be tau(B-)/tau(B0) =
1.110 +/- 0.056 +0.033 -0.030, where the first uncertainties are statistical
and the second are systematic.
