[en] The relativistic equation of motion of an electron in the presence of a very large amplitude electromagnetic wave and a uniform magnetic field is solved. Dispersion relation for the wave propagation is obtained and its dependence on the wave amplitude is studied. It is shown that in the electron cyclotron frequency range, the large amplitude waves have smaller guidance angle as compared to the waves of negligible amplitudes. The results are discussed, qualitatively, for the electromagnetic wave propagation in pulsar magnetosphere. (author). 11 refs