Twenty-two marine-type heat flow determinations in Yellowstone Lake indicate a rapid transition from high heat flow values outside the Yellowstone caldera, 100--300 mW m-2 (2.5--7.5 μcal cm-2 s-1), to an extensive area of very high heat flow, 600--700 mW m-2 (14.5 μcal cm-2 s-1) and greater, within the caldera. The thermal transition occurs 5--10 km inside the mapped caldera boundary and approximately coincides with the point at which a zone of earthquake activity beginning outside the caldera terminates. Higher heat flow in West Thumb and Mary Bay (about 1600 mW m-2 (40 μcal cm-2 s-1)) outlines two thermal subprovinces within the caldera. The conductive heat loss beneath the lake is similar in magnitude to the average convective heat loss calculated from chloride budgets. Extrapolated temperatures beneath the lake reach the boiling point under pressure at approximately 500-m depth, and deeper temperatures probably increase along the boiling point curve in a laterally extensive geothermal reservoir to depths of the order of 800¿400 m. Thermal areas in the park probably result from structurally and topographically controlled convection systems in this surficial system. A deeper, hotter fluid circulation system in fractures in the solidified granite postulated to underly the caldera at shallow depths (about 1 km) may exist at depth. The high heat flow is derived from a shallow cooling batholith beneath the caldera.