I n honour of Prof. Dr. J. AUTH'S 50th birthday I n a semiconducting bismuth-ant,imony alloy, the propagation of electromagnetic waves is observed in the frequency range between 35 and 70 GHz and far temperatures of liquid helium in the total angular range between the Voigt, a.nd the Faraday configuration. The comparison with the theoretical dispersion behaviour, where a conductivity tensor corresponding t o a tripartite ellipsoidalFermi surface is utilized, allows a qualitative understanding of the wave propagation. The influence of the redistribution effects of the charge carriers among the individual parts of the Fermi surface in a quantizing magnetic field is essential, and the inaccurate knowledge of the energy level structure makes the comparison between experiment and theory difficult.