  The naturalness of maximal mixing between myon- and tau-neutrinos is
investigated. A spontaneously broken nonabelian generation symmetry can explain
a small parameter which governs the deviation from maximal mixing. In many
cases all three neutrino masses are almost degenerate. Maximal
ny_my-ny_tau-mixing would indicate that the leading contribution to the light
neutrino masses arises from the expectation value of a heavy weak triplet
rather than from the seesaw mechanism. In this scenario the deviation from
maximal mixing is predicted to be less than about 1%.
