Using oceanographic heat-flow techniques, 162 measurements of heat flow were made in Lake Superior during the summers of 1966 and 1967. These data are of high quality, with precisions with respect to intercomparisons typically in the 3–5% range. The data define two very clear features. One is a trough of low heat-flow values, which runs continuously for 650 km along the northern edge of the lake, with values ranging between 0.46 and 0.98 heat-flow units (HFU) (19.2–41.0 mW/m2). This feature correlates with surface exposure of Keweenawan mafic volcanics; it is believed to delineate a major crustal separation associated with the Midcontinent Rift and is filled to crustal thicknesses with mafic intrusives and extrusives. This feature has not been imaged with the seismic reflection profiling of GLIMPCE. The other heat-flow feature is an arcuate ridge of high heat-flow values (1.0–1.45 HFU; 41.8–60.7 mW/m2), parallel to and south of the heat-flow trough. The highest areas of this ridge correspond to areas of thick rift-filling Keweenawan sediments. The high heat flow is modulated to lower values in areas where the thick sediments overlie highly thinned crust now containing large thicknesses of mafic volcanic rock. The heat-flow features show very good correlation with the magnetic anomaly map of Lake Superior, but only spotty correlation with the Bouguer gravity anomaly features.