Using OPAL data collected at the LEP collider from 1990 to 1995, the
branching ratio of the $\tau \rightarrow \mu \nu_\tau \nu_\mu$ decay has
been measured.  The branching ratio is measured in two largely independent
ways, using data from different subdetectors within the OPAL detector.
While one method uses the tracking chambers and muon chambers, the other
method uses calorimeter information. The second method is used as a
cross-check on the results from the first method. The result is used in
conjunction with other measurements to test lepton universality.