  The first observation of the decay Bs -> D0bar K- {\pi}+ is reported. The
analysis is based on a data sample, corresponding to an integrated luminosity
of 1.0 fb-1 of pp collisions, collected with the LHCb detector. The branching
fraction relative to that of the topologically similar decay B0 -> D0bar {\pi}+
{\pi}- is measured to be BR(Bs -> D0bar K- {\pi}+)/BR(B0 -> D0bar {\pi}+
{\pi}-) = 1.18 +- 0.05 (stat.) +- 0.12 (syst.). In addition, the relative
branching fraction of the decay B0 -> D0bar K+ {\pi}- is measured to be BR(B0
-> D0bar K+ {\pi}-)/BR(B0 -> D0bar {\pi}+ {\pi}-) = 0.106 +- 0.007 (stat.) +-
0.008 (syst.).
