The indoor (basement) radon concentrations and the uranium, organic carbon, and radon contents of samples from the underlying Ohio Shale (Upper Devonian) were investigated in six approximately 100 square kilometer areas of Ashtabula, Cuyahoga, Erie-Huron, Franklin, Pike, and Logan counties. The thickness and lithology of the sediment overburden above the Ohio Shale in these areas also was investigated. Results show that the amount of radon gas emanating from the Ohio Shale is a direct function of the uranium concentration in this geologic unit and that the uranium itself is ultimately controlled by the amount of organic matter. Uranium, organic matter (as measured by total organic carbon), and radon in the Ohio Shale outcrops all increase in a westward direction across Ohio. Similarly, the indoor radon concentration in houses increases from east to west across the state. Where the average thickness of the sediment overburden exceeds 27 m (Franklin and Logan counties), much of the radon in houses may be derived from Ohio Shale clasts in the glacial till. In other areas where the average overburden thickness is 6 m or less (Ashtabula, Cuyahoga, Erie-Huron, and Pike counties), the indoor radon levels appear to be directly controlled by emanations from the underlying Ohio Shale bedrock.