  In a supersymmetric model with the lightest supersymmetric particle (LSP)
$\chi$ in the range of a few hundred MeV's, the decay $\tau {\longrightarrow}
\mu \chi \chi$ is going to be allowed. We investigate the departure from
tau-universality caused by this decay. It is found that the universality
violation in this way can be greater than both non-universal electroweak
radiative corrections and supersymmetric one-loop corrections over a
considerable region of the parameter space allowed by experiments so far. Thus
it suggests a method of constraining the parameter space with light LSP's using
data from tau-factories.
