  For a Radon measure $\mu$ on $\bbR,$ we show that $L^{\infty}(\mu)$ is
invariant under the group of translation operators $T_t(f)(x) = {$f(x-t)$}\ (t
\in \bbR)$ if and only if $\mu$ is equivalent to Lebesgue measure $m$. We also
give necessary and sufficient conditions for $L^p(\mu),\1 \leq p < \infty,$ to
be invariant under the group $\{T_t\}$ in terms of the Radon-Nikodym derivative
w.r.t. $m$.
