The geothermal resource of southern Idaho as assessed by the U.S. Geological Survey in 1978 is large. Most of the known hydrothermal systems in southern Idaho have calculated reservoir temperatures of less than 150 C. Water from many of these systems is valuable for direct heat applications. A majority of the known and inferred geothermal resources of southern Idaho underlie the Snake River Plain. However, major uncertainties exist concerning the geology and temperatures beneath the plain. The largest hydrothermal system in Idaho is in the Bruneau-Grang View area of the western Snake River Plain with a calculated reservoir temperature of 107 C and an energy of 4.5 x 10 to the 20th power joules. No evidence of higher temperature water associated with this system was found. Although the geology of the eastern Snake River Plain suggests that a large thermal anomaly may underlie this area of the plain, direct evidence of high temperatures was not found. Large volumes of water at temperatures between 90 and 150 C probably exist along the margins of the Snake River Plain and in local areas north and south of the plain.