  We consider the role of a propagation effect such as the stimulated Raman
scattering on the polarization of radio pulses. When an intense electromagnetic
wave with frequency close to the plasma frequency interacts with the plasma in
the pulsar magnetosphere, the incident wave undergoes stimulated Raman
scattering. Using typical plasma and magnetic field parameters, we compute the
growth rate and estimate the polarization properties of the scattered mode. At
some conditions, we find that the polarization properties of the scattered mode
can become significantly different from those of the incident wave. The
frequencies, at which strong Raman scattering occurs in the outer parts of
magnetosphere, fall into the observed radio band.
  In some pulsars, for example, PSR B0628-28 and 1914+13, a large and symmetric
type of circular polarization has been observed. We propose that such an
unusual circular polarization is produced by the propagation effects.
