The problem of mode conversion and scattering by a finite‐length section of magnetoplasma‐filled waveguide is formulated in matrix notation. This formulation becomes increasingly exact as the dimensions of the matrices (i.e., the number of modes considered) increases. The desired reflection‐ and transmission‐coefficient matrices are readily evaluated with a digital computer. Computations of TE010‐mode reflection coefficients indicate that good accuracy can be obtained with a fairly small (∼10) number of modes. Large errors result from considering only the incident (TE010) mode, however.