  We report the first observation of $B^0 \to D_s^+ \pi^-$ and an improved
measurement of ${\bar B^0} \to D_s^+K^-$ based on $274 \times 10^6$ $B{\bar B}$
events collected with the Belle detector at KEKB. We measure the branching
fractions ${\cal B}({\bar B^0} \to D_s^+K^-) = (2.93 \pm 0.55 \pm 0.79) \times
10^{-5}$ and ${\cal B}(B^0 \to D_s^+ \pi^-) = (1.94 \pm 0.47 \pm 0.52) \times
10^{-5}$.
