Properties of electromagnetic propagation at extremely low frequencies (below 500 cycles) are deduced from analysis of pulse distortion. The source of the transmitted pulse is lightning. Two distinct transmission types are analyzed. Normal, or type I, propagation is shown to behave as if the earth's surface and ionosphere constitute a wave guide composed of two plane parallel surfaces, with one surface partially conducting. This wave guide has no low‐frequency limit on progagation; attenuation formulas are given. The transmission properties of type II, or anomalous propagation, are derived and seen to be inexplicable by means of this wave‐guide model. The analysis leads to a determination of the effective conductivity of the ionospheric reflector at these frequencies: the daytime reflecting layer (Dlayer) conductivity is 104esu; the nighttime conductivity (Elayer) is 5×104esu; both results are far lower than deduced by other means.