  A combined mass and particle identification fit is used to make the first
observation of the decay Bs --> Ds K and measure the branching fraction of Bs
--> Ds K relative to Bs --> Ds pi. This analysis uses 1.2 fb^-1 integrated
luminosity of pbar-p collisions at sqrt(s) = 1.96 TeV collected with the CDF II
detector at the Fermilab Tevatron collider. We observe a Bs --> Ds K signal
with a statistical significance of 8.1 sigma and measure Br(Bs --> Ds K)/Br(Bs
--> Ds pi) = 0.097 +- 0.018(stat) +- 0.009(sys).
