Numerous thermal areas are found on the southwest flank of the active volcano Rincón de la Vieja, Costa Rica. These are part of a larger band of thermal areas that parallels the volcanic front for about 25 km. A total of 318 soil Hg samples were collected on the southwest flank of the volcano, predominantly at the Las Pailas hot springs. Soil Hg concentrations range from 6 to 180,000 ppb and can be divided into three populations with threshold values of 47 and 190 ppb. Aureole Hg values are widely distributed; anomalous soil Hg concentrations were found at four locations. Two-dimensional Fourier analysis of smoothed Hg values from the Las Pailas thermal area reveals a N70W trending lineament of high Hg concentration. This lineament is thought to indicate the presence of a fault along which upwelling of geothermal fluids is enhanced. The orientationof this fault coincides with the orientation of volcano alignments and structural features identified elsewhere in the region.