Using data collected with the L3 detector near the Z resonance, corresponding to an integrated luminosity of 150 pb−1, the branching fractions of the tau lepton into electron and muon are measured to be B(τ → eν ̄eντ ) = (17.806 ± 0.104 (stat.) ± 0.076 (syst.))%, B(τ → μν ̄μντ ) = (17.342±0.110 (stat.)±0.067 (syst.))%. From these results the ratio of the charged current coupling constants of the muon and the electron is determined to be gμ/ge = 1.0007 ± 0.0051. Assuming electron-muon universality, the Fermi constant is measured in tau lepton decays as GF = (1.1616± 0.0058) × 10−5 GeV−2. Furthermore, the coupling constant of the strong interaction at the tau mass scale is obtained as αs(m2 τ ) = 0.322 ± 0.009 (exp.) ± 0.015 (theory).Facultad de Ciencias Exactas