  The totality of present neutrino data seems to require four light neutrinos,
but only three of them can be the neutral components of left-handed lepton
doublets. To accommodate one or more naturally light singlet neutrino(s), an
extra U(1) gauge factor is proposed to implement an analogous seesaw mechanism
which accounts for the light doublet neutrinos. Using the constraints of
anomaly cancellation, the property of this U(1) is determined. The most
attractive theoretical framework is that of a supersymmetric $SU(3)_C \times
SU(2)_L \times U(1)_Y \times U(1)_N$ model already proposed.
