Using interactions of 350 GeV/c pi(-) in copper and tungsten targets, the b
ranching fraction for the leptonic decay B(D-s --> mu v(mu)) is measured to
he (0.83 +/- 0.23(stat.)+/- 0.06 +/- 0.18(B(Ds --> phi pi)))%. The resulti
ng value for the D-s decay constant is f(D,) = 323 +/- 44 +/- 12 +/- 34. (C
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