We report a measurement of the semimuonic decay D0-->K-mu+nu from data
taken during the 1987-1988 fixed target run at Fermilab by the E687 C
ollaboration. We obtain D0-->K-mu+nu)/GAMMA(D0-->K-pi+)=0.82+/-0.13+/-
0.13 and use this result to calculate >K-mu+nu)/GAMMA(D0-->K-mu+nu)=0
.59+/-0.10+/-0.13.