Evidence has been presented by several investigators indicating the possibility that radio waves penetrate sufficiently deep into the earth to be useful in prospecting for oil. Conventional electromagnetic theory used with normal values of the earth constants indicates, however, that the attenuation is too great to allow the signal to be used after propagation through significant distances of shale.To settle the above question an experiment was conceived and carried out in which the signal level in the earth was measured at various distances from a battery-operated transmitter operating at 1,652 kc suspended in an uncased, mud-filled hole by a mud-saturated rope. The mud resistivity was matched to that of the 40 foot thick shale section used in order that the entire immediate region would be as nearly as possibly homogeneous. The receiver was also battery-operated and suspended by a cable in other similar mud-filled holes at various distances from the first hole. The attenuation constant in shale was found to be 0.231 nepers/foot (2 db/foot), which is much too large to give any hope of deep penetration. A few measurements in a limestone section gave a value of 0.086 nepers/foot, which is also too large to be useful. Although these values are quite high, they are lower than theory predicts for these earth resistivities by a factor of about two.