Symmetry Between Muon and Electron

Muon-electron symmetry is treated and a definition of formal operation of this symmetry is given. It is shown that the total lagrangian excluding weak and electromagnetic couplings, can be written in a form exhibiting such a symmetry. This symmetry cannot be satisified when universal weak interactions are included for one neutrino but is satisfied by a two-neutrino theory. The close connection between this symmetry and the principle forbidding muon transformation into electrons is pointed out. (D.L.C.)