The τ− → µ−ν¯µντ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B(τ− → µ−ν¯µντ ) = 0.1734 ± 0.0009(stat) ± 0.0006(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios gµ/ge = 1.0005 ± 0.0044 and gτ /ge = 1.0031 ± 0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η = 0.004 ± 0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, mH± > 1.28 tan β, in the Minimal Supersymmetric Standard Model.