  We have observed $23.2 \pm 6.0_{-0.9}^{+1.0}$ purely-leptonic decays of
$D_s^+ -> \mu^+ \nu_\mu$ from a sample of muonic one prong decay events
detected in the emulsion target of Fermilab experiment E653. Using the $D_s^+
-> \phi \mu^+ \nu_\mu$ yield measured previously in this experiment, we obtain
$B(D_s^+ --> \mu^+ \nu_\mu) / B(D_s^+ --> \phi \mu^+ \nu_\mu) =0.16 \pm 0.06
\pm 0.03$. In addition, we extract the decay constant $f_{D_s}=194 \pm 35 \pm
20 \pm 14 MeV$.
