A measurement of the Michel parameters and the average ντ helicity in τ lepton decays is described. The data was collected with the ALEPH detector at LEP during the years 1991 to 1995. A total integrated luminosity of 155 pb−1 is analysed. The Michel parameters ρl,ξl,(ξδ)l(l=e,μ) , and ημ are determined for the leptonic decays, and the chirality parameters ξπ,ξρ , and ξa1 for the hadronic final states. Under the assumptions of e- μ universality and ξπ=ξρ=ξa1 , the values ρl=0.742±0.016,ηl=0.012±0.026,(ξδ)l=0.776±0.051,ξl=0.986±0.074 , and ξh=0.992±0.011 are obtained. No significant deviation is observed from the Standard Model assumption of the V-A structure of the charged weak interaction.