Although the theories of electromagnetic modeling are well understood, the basic computational problems are practical, involving the quadratically growing requirements on computer storage and cubically growing computation time with the number of cells required to discretize the modeling body. There are a number of approaches aimed at reducing these requirements by making use of the symmetry properties of the scattering body and the exciting source. This note will show that the reduced scattering matrix of a twofold-symmetric structure under plane wave excitation is also symmetric if the structure is discretized with equal-size cells, and hence the total matrix storage requirement can be reduced by 1/16 and the matrix factorization time by approximately 1/64. Thus, this method is superior to the block diagonalization method in this special case. -from Author