  We report the first observation of a B meson decay that is not accessible by
a direct spectator process. The channel B0bar -> Ds+ K- is found in a sample of
85 timse 10^6 B Bbar events, collected with the Belle detector at KEKB, with a
branching fraction Br(B0bar -> Ds+ K-)=(4.6^{+1.2}_{-1.1} +- 1.3) times
10^{-5}. We also obtain evidence for the B0 -> Ds+ pi- decay with branching
fraction Br(B0 -> Ds+ pi-)=(2.4^{+1.0}_{-0.8} +- 0.7) times 10^{-5}. This value
may be used to extract a model-dependent value of |Vub|.
