When a static magnetic field is applied in the longitudinal direction to a wave guide containing a ferrite material, the medium within the guide becomes anisotropic. It has been shown by Polder that when a microwave field is propagating within such a medium, the relation between the components of B and H is given by: Bx=μHx-jμ'Hy,By=jμ'Hx+μHy,Bz=Hz, where μ and μ' depend on the static magnetic field. Using these equations, an investigation of the wave-guide modes is made. Formal solutions are obtained for guides having circular symmetry. These solutions depend on the solution of some rather involved eigenvalue equations which determine the propagation constants, and which probably can be solved only by numerical methods.