  The recent observation of the purely leptonic decay B^\pm\to \tau^\pm\nu_\tau
at the B factories permits a sizeable contribution from a charged Higgs boson
(H^\pm). Such a H^\pm would also contribute to the decays D^\pm_s\to
\mu^\pm\nu_\mu and D^\pm_s\to \tau^\pm\nu_\tau, which are being measured with
increasing precision at CLEO-c. We show that the branching ratios of D^\pm_s\to
\mu^\pm\nu_\mu and D^\pm_s\to \tau^\pm\nu_\tau could be suppressed by up to 10%
from the Standard Model prediction, which is larger than the anticipated
precision in the measurements of these decays at forthcoming BES-III.
