  We report a measurement of the exclusive $B^+$ meson decay to the final state
$D_s^- K^+\pi^+$ using $520 \times 10^{6} B\bar{B}$ pairs collected near the
$\Upsilon(4S)$ resonance, with the Belle detector at the KEKB asymmetric-energy
$e^+e^-$ collider. Using the $D_S^- \to \phi \pi^-$ decay mode to reconstruct
$D_s^-$ mesons, we obtain the branching fraction ${\cal B}(B^+\to
D_s^-K^+\pi^+)= (1.77^{+0.12}_{-0.12} {\mathrm (stat)} \pm 0.16 {\mathrm
(syst)} \pm 0.23 {\cal(B)})\times 10^{-4}$. We also present preliminary results
of a study of the two-body $D_sK$, $D_s\pi$ and $K\pi$ subsystems observed in
this final state.
