In this paper we consider the propagation of electromagnetic wave in a very rarefied plasma, on the assumption that the magnetic field determines an electric anisotropie and the dielectric power is a tensor with parallel and perpendicular component to the magnetic field. We establish the differential equation of the wave surface and the equation that define the velocity of propagation; we give of the last an approximate solution, and finally we consider the significant case in which the magnetic field is tangent to the wave surface. In this case the velocity of propagation of the wave front is dependent on the normal component to the wave surface of the Poynting vector. Another interesting case is that in which the parallel component to the magnetic field of the electric field, and the perpendicular component, have the same projection on the normal to the wave surface.