  We have observed the decay of the $B^-$ meson into $D^0 K^-$, using 3.3
million $B\bar B$ pairs collected with the CLEO II detector at the Cornell
Electron Storage Ring. We find the branching fractions of this decay to be
suppressed relative to the decay into $D^0 \pi^-$ by $0.055 \pm 0.014 \pm
0.005$.
