Aperture fields in the diffraction by a slit was studied numerically and experimentally. Numerical calculations are based on the exact solution in the form of an infinite series of Mathieu functions. Experimental measurements, by means of a parallel‐plate device, of the field distribution over the aperture in slits of 3.5&lgr;, 3&lgr;, 2.21&lgr;, 1.27&lgr; width (&lgr;=wavelength) are discussed, and show satisfactory agreement with the exact solution for slits of width 1.27&lgr; and 2.21&lgr;. The exact solution of diffraction by a slit due to a line source is also presented in the Appendix.