  In 60 pb$^{-1}$ of data taken on the $\psi(3770)$ resonance with the CLEO-c
detector, we find 8 $D^+\to\mu^+\nu$ event candidates that are mostly signal,
containing only 1 estimated background. Using this statistically compelling
sample, we measure the value of ${\cal{B}}(D^+\to\mu^+\nu)=(3.5\pm 1.4 \pm
0.6)\times 10^{-4}$, and determine $f_{D^+}=(198\pm 41\pm 17)$ MeV.
