The polarisation of τ '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 Ae=0.1504±0.0068 and Aτ=0.1451±0.0059 which are consistent with the hypothesis of e- τ universality. Assuming universality, the value Ae-τ=0.1474±0.0045 is obtained from which the effective weak mixing angle sin2θeffW=0.23147±0.00057 is derived.