The amount of 222Rn that enters a house depends to a large extent on the concentration of radon's parent, 226Ra in the soil. With the assumption that radium is in radioactive equilibrium with its ultimate parent, 238U, we used to National Aerial Radiometric Reconnaisance (NARR) data and a database covering the concentration and distribution of of uranium in California rocks to estimate the regional distribution of radium. A north-to-south increase in radium occurs between 42¿ and 36¿N, then radium decreases southward. This pattern is explained by the distribution of rock types. The overall mean radium concentrations estimated from the aeroradiometric and lithologic data are similar, but there is a significant discrepancy between aeroradiometric and lithologic-estimated radium in central California. Ground measurements suggest that the radium values from aeroradiometric measurements there are erroneously high, pointing out the necessity of verifying aeroradiometrically-determined data before they are used to help predict radon production. ¿ American Geophyscial Union 1990