  First observation of the decay Bs -> D0bar phi is reported using pp collision
data, corresponding to an integrated luminosity of 1.0 fb^-1, collected by the
LHCb experiment at a centre-of-mass energy of 7 TeV. The significance of the
signal is 6.2 standard deviations. The branching fraction is measured relative
to that of the decay Bs -> D0bar K*0bar to be 0.069 +/- 0.013 (stat) +/- 0.007
(syst). The first measurement of the ratio of branching fractions for the
decays Bs -> D0bar K*0bar and Bd->D0bar K*0 is found to be 7.8 +/- 0.7 (stat)
+/- 0.3 (syst) +/- 0.6 (f_s/f_d), where the last uncertainty is due to the
ratio of the Bs and Bd fragmentation fractions.
