  The decay $\tau^{-}\to K^{-}K_S\nu_{\tau}$ has been studied using
$430\times10^6$ $e^+e^-\to \tau^+\tau^-$ events produced at a center-of-mass
energy around 10.6 GeV at the PEP-II collider and studied with the BABAR
detector. The mass spectrum of the $K^{-}K_S$ system has been measured and the
spectral function has been obtained. The measured branching fraction ${\cal
B}(\tau^{-}\to K^{-}K_S\nu_{\tau}) = (0.739\pm 0.011(\rm stat.)\pm 0.020(\rm
syst.))\times 10^{-3}$ is found to be in agreement with earlier measurements.
