  We study the exactly degenerate case for three lefthanded Majorana neutrinos
building a general parametrization for the leptonic mixing matrix characterized
by two angles and one CP violating phase and identify a weak-basis invariant
relevant, in this case, for CP violation. After lifting the degeneracy, this
parametrization accommodates the present data on atmospheric and solar
neutrinos, as well as neutrinoless double beta decay. Some of the leptonic
mixing ans\"atze suggested in the literature correspond to special cases of
this parametrization.
