A soil mercury survey was conducted near the town of Calistoga, California to identify and delineate a buried fault system that is thought to control the upwelling of low-to-moderate temperature geothermal fluids in the upper Napa Valley. Soil samples were collected at 100 m intervals along traverses that crossed hot springs and existing geothermal well sites. Strong mercury anomalies occur along a broadly-defined zone and in close proximity to surface thermal activity including active hot springs and silica sinter formations. In contrast, background mercury concentrations are present in locations with little or no indication of subsurface thermal activity, such as along the margins of the valley or near groundwater wells producing non-thermal water. Analysis of smoothed Hg values reveals a N65W-trending lineament of high Hg concentrations. These results suggest that soil mercury surveys can be a useful and cost-effective method for the identification and mapping of structures controlling subsurface fluid flow in low-to-moderate temperature geothermal systems.