  The oscillations of pseudo-Dirac neutrinos in matter are discussed and
applied to the solar neutrino problem. Several scenarios such as both $\nu_e$
and $\nu_{\mu}$ being pseudo-Dirac and only $\nu_e$ or $\nu_{\mu}$ being
pseudo-Dirac are examined. It is shown that the allowed region in the
mass-mixing angle parameter space obtained by comparing the solar neutrino data
with the calculations based on the standard solar model and the MSW effect is
not unique. The results depend on the nature of neutrinos; for example, if both
$\nu_e$ and $\nu_{\mu}$ are pseudo-Dirac, the allowed region determined by the
current solar neutrino data does not overlap with that obtained in the usual
case of pure Dirac or Majorana neutrinos.
