  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
preliminary values of B(D+ to mu+ nu) = (3.5 +- 1.4 +- 0.6)*10^{-4}, and
determine f_{D+} =(201+- 41+- 17) MeV.
