  The polarisation of $\tau$'s produced in Z decay is measured using 160
pb$^{-1}$ of data accumulated at LEP by the ALEPH detector between 1990 and
1995. The variation of the polarisation with polar angle yields the two
parameters ${\cal A}_e = 0.1504 \pm 0.0068 $ and ${\cal A}_{\tau} = 0.1451 \pm
0.0059$ which are consistent with the hypothesis of $e$-$\tau$ universality.
Assuming universality, the value ${\cal A}_{e{-}\tau} = 0.1474 \pm 0.0045$ is
obtained from which the effective weak mixing angle $\sin^2
{\theta_{\mathrm{W}}^{\mathrm{eff}}} =0.23147 \pm 0.00057 $ is derived.
