  Using 230.2 fb^(-1) of e+e- annihilation data collected with the BABAR
detector at and near the peak of the Y(4S) resonance, 489 +/- 55 events
containing the pure leptonic decay D_s^+ --> mu^+ nu_mu have been isolated in
charm-tagged events. The ratio of partial widths Gamma(D_s^+ --> mu^+
nu_mu)/Gamma(D_s^+ --> phi pi^+) is measured to be 0.143 +/- 0.018 +/- 0.006
allowing a determination of the pseudoscalar decay constant f_{D_s} = (283 +/-
17 +/- 7 +/- 14) MeV. The errors are statistical, systematic, and from the
D_s^+ --> phi pi^+ branching ratio, respectively.
