A method is described for measuring the radium content of natural waters. The method is shown to be accurate to within a few percent for concentrations as low as 100×10−12g Ra per liter. Results are given of measurements on several connate waters from mid‐continent oil fields. A correlation between radium and calcium content of the waters is indicated. Samples taken from the same wells over a period of several months indicate that the radium concentration is a definite property of a connate water.