The paper describes a method of measuring the a.c. resistivity of the earth based on the Carson-Pollaczek theory for the mutual impedance of earth-return circuits. The method consists in the measurement of the e.m.f.'s induced in a search coil placed at various distances from an earthreturn circuit carrying alternating current. The experimental points are superimposed on a set of theoretical curves calculated for different values of resistivity, and the curve with which the points coincide gives the resistivity.Where a power line exists in the neighbourhood of the site whose resistivity is to be measured, one of the zero phasesequence harmonics can be used as the inducing current.The method has been used to measure the resistivity of the earth at various sites in England and Italy, and the results obtained are given together with particulars of the geological formation at the sites.