We consider the role of a propagation effect such as stimulated Raman scatt
ering 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 m
ode. Under some conditions, we find that the polarization properties of the
scattered mode can become significantly different from those of the incide
nt wave. The frequencies at which strong Raman scattering occurs in the out
er parts of the magnetosphere fall into the, observable radio band. In some
pulsars, for example PSR B0628-28 and B1914+13, a large and symmetric type
of circular polarization has been observed, We propose that such an unusua
l circular polarization is produced by the propagation effects.