We present the result from a precision measurement of the mass of the $\tau$ lepton, $M_\tau$, based on $423~fb^{-1}$ of data recorded at the $\Upsilon(4S)$ resonance with the BaBar detector. Using a pseudomass endpoint method, we determine the mass to be $1776.68 \pm 0.12 (stat) \pm 0.41 (syst)~MeV$. We also measure the mass difference between the $\tau^ $ and $\tau^-$ , and obtain $(M_{\tau^{ }}-M_{\tau^{-}})/M^\tau_{AVG} ...