Models of neutrino masses are discussed capable of explaining
in a natural way the maximal mixing between νμ and
ντ observed by the Super-Kamiokande collaboration.
For three generations of leptons two
classes of such models are found implying:
a) Δm232≪Δm122≈Δm132
and a small mixing between νe and the other two neutrinos,
b) Δm122≪Δm132≈Δm232
and a nearly maximal mixing for solar neutrino oscillations in vacuum.