Search for the decay D+'+ and an upper limit on the pseudoscalar decay constant

We report the results of a search for the leptonic decay D/sup +/..--> mu../sup +/..nu../sub ..mu../ using the Mark III detector at the SLAC e/sup +/e/sup -/ storage ring SPEAR. A data sample of 9.3 pb/sup -1/ collected at the psi(3770) resonance yields no signal events, corresponding to a 90%-confidence-level upper limit of 7.2 x 10/sup -4/ on the branching ratio B(D/sup +/..--> mu../sup +/..nu../sub ..mu../). This represents an upper limit on the pseudoscalar decay constant f/sub D/ of 290 MeV/c/sup 2/.