Soil mercury concentrations from Yellowstone National Park were determined using a thin gold film mercury detector. Reconnaissance sampling along roads and trails using a 1.6-km-sample interval revealed narrow soil Hg anomalies that are restricted to thermal regions and are surrounded by large areas with lower levels of soil mercury. Background levels of mercury in Yellowstone soils are 20 ppb with an anomaly threshold of 40 to 50 ppb. Detailed sampling on a 50-m grid was carried out in one vapor-dominated and three water-dominated areas. Although two of the water-dominated areas contain anomalous concentrations of soil Hg, there is no direct correlation between the Hg anomalies and the presence of hot springs. The third hot water area, the Mammoth Hot Springs, contains no significant soil Hg anomaly. In the vapor-dominated system there is a strong correlation between soil Hg anomalies and intense vent activity. Partitioning of mercury into the vapor phase during subsurface boiling and subsequent separation of the vapor phase from the thermal waters are believed to be the major controls on the formation of soil Hg anomalies at Yellowstone. Geothermal exploration using soil Hg concentrations could be carried out on a local scale in Yellowstone but because of the narrow Hg anomalies would not be feasible on a regional scale.