A method is presented for solving the inverse magnetotelluric problem. By means of a transformation of variables, the earth is modeled at low frequencies as a lossless, nonuniform transmission line. The approach described utilizes the realizability and synthesis theory available for this class of lines. As a result, the numerical calculations reduce to the solution of an approximation problem involving a rational function of ω1/2, where ω is the angular frequency. The remaining computations are straightforward and lead to a unique determination of the conductivity as an implicit function of depth.