  CP violation in the decays $t \to \ell^+\nu b$ and $\bar t \to \ell^-\bar\nu
\bar b$ from the production process $e^-e^+ \to \ttbar$ is discussed. Since the
asymmetry proposed as a measure of CP violation vanishes even at the one-loop
level in the Standard Model (SM), it may be a useful tool to search for sources
of CP violation outside of the SM. As an illustration the asymmetry has been
computed within supersymmetric extensions of the SM. Prospects for its
measurement at future high-energy linear colliders are discussed.
