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We report an observation of the decay Bs0 → Ds-π+ in pp̅ collisions at √s = 1.96 TeV using 115 pb-1 of data collected by the CDF II detector at the Fermilab Tevatron. We observe 83 ± 11(stat) Bs0 → Ds -π+ candidates, representing a large increase in statistics over previous measurements and the first observation of this decay at a pp̅ collider. We present the first measurement of the relative branching fraction Ɓ(Bs0 → Ds-π+)/ Ɓ(B0 → D- π+)= 1.32 ± 0.18(stat) ± 0.38(syst). We also measure Ɓ(B+ →D0 π+)/ Ɓ(B0 → D- π+) = 1.97 ± 0.10(stat) ± 0:21(syst), which is consistent with previous measurements.