INTRODUCTION: The Indiana State Board of Health, with assistance from the U.S. Environmental Protection Agency (EPA), expanded the program to assess indoor-radon levels in homes in the state during the 1987-1988 heating season. Radon is a naturally occurring gaseous radioisotope in the uranium-238 decay chain. Because radon is released from bedrock and unconsolidated materials containing the parent isotopes, the first step in conducting an indoor-radon survey was the geologic characterization of Indiana. In July 1987 the EPA requested assistance from the Indiana Geological Survey in preparing a geologic characterization of the state. This report fulfills that request; it includes brief descriptions of bedrock and unconsolidated materials exposed at or near the surface in Indiana, divides the state into eight geologic regions, and describes the relative radon-potential of areas within those regions. At the request of the EPA, areas within Indiana were given radon-potential ratings of low, low/moderate, moderate, moderate/high, or high. The radon-potential ratings shown in plate 1 are based on the composition of the bedrock and unconsolidated material, soil permeability, and uranium anomalies shown on the National Uranium Resource Evaluation (NURE) uranium-anomaly interpretation maps (EG&G geoMetrics, 1980a, 1980b, 1981a-f; Geodata International, Inc, 1980?; Texas Instruments Inc., 1979). The uranium-anomaly data were gathered during the National Airborne Radiometric Reconnaissance part of the NURE program. Within many counties areas of differing radon potential exist.