  We consider the polarization behaviour of radio waves propagating through an
ultrarelativistic highly magnetized electron-positron plasma in a pulsar
magnetosphere. The rotation of magnetosphere gives rise to the wave mode
coupling in the polarization-limiting region. The process is shown to cause
considerable circular polarization in the linearly polarized normal waves.
Thus, the circular polarization observed for a number of pulsars, despite the
linear polarization of the emitted normal waves, can be attributed to the
limiting-polarization effect.
