To interpret the magneto‐telluric sounding data in terms of layering in the subsurface, one needs a catalog of “standard” curves. The purpose of this paper is to present such a catalog for the three‐layer cases. The calculations were done by incorporating the formulas given by Cagniard (1953) into a digital computer program. The catalog consists of 117 apparent‐resistivity‐vs‐period curves representing ten resistivity combinations. In each case the third, semi‐infinite medium represents the “basement” with infinite resistivity. In addition, a set of two‐layer curves for the total range of resistivity combinations is also given. The procedure in using the curves is briefly explained.