  We study mixing and CP violation of three left-handed Majorana neutrinos in
the limit of exactly degenerate masses, identify the weak-basis invariant
relevant for CP violation and show that the leptonic mixing matrix is
parametrized only by two angles and one phase. After the lifting of the
degeneracy, this parametrization accommodates the present data on atmospheric
and solar neutrinos, as well as double beta decay. Some of the leptonic mixing
ansatze suggested in the literature correspond to special cases of this
parametrization.
