  We report a measurement of the branching fraction of the semileptonic decay
B- -> D0 l- nubar_l (and its charge conjugate) using 62.8/fb of Upsilon(4S) ->
B Bbar data recorded by the Belle II experiment at the SuperKEKB
asymmetric-energy e+ e- collider. The neutral charm meson is searched for in
the decay mode D0 -> K- pi+ and combined with a properly charged identified
lepton (electron or muon) to reconstruct this decay. No reconstruction of the
second B meson in the Upsilon(4S) event is performed. We obtain Br( B- -> D0 l-
nubar_l ) = (2.29 +/- 0.05(stat) +/- 0.08(syst))%, in agreement with the world
average of this decay. We also determine the ratio of the electron to muon
branching fractions to be R(e/mu) = 1.04 +/- 0.05(stat) +/- 0.03(syst) and
observe no deviation from lepton universality.
