Using about 3.9 million hadronic Z decays 
from ${\rm e}^+{\rm e}^-$ collisions recorded by
the OPAL detector at LEP 
at centre-of-mass energies $\sqrt{s}\approx M_{\rm Z}$,
the branching ratio for the 
decay $\rm D_s^-\rightarrow\tau^-\bar{\nu}_\tau$ has been measured to be
$$\rm BR(D_s^-\rightarrow\tau^-\bar{\nu}_\tau) =
(7.0\pm 2.1 (stat) \pm 2.0 (syst))\%.$$
This result can be used to derive the
decay constant of the $\rm D_s^-$ meson:
$$\rm f_{D_s} = (286\pm 44 (stat) \pm 41 (syst)) MeV.$$