Heat flow studies in southern Africa reveal a pattern of low heat flow in Archean cratons compared with Proterozoic mobile belts that provides grounds for modelling in which cratons have lower mantle heat flux and greater lithospheric thickness. There is little detailed information regarding thermal transitions across craton boundaries but new data from Lesotho provide insights into the nature of the boundary between the Kaapvaal craton and the Natal belt. Measurements at nine localities in a 90-km-long north-south traverse show an increase of heat flow from typically cratonic values of about 45 mW m-2 in the north to about 80 mW m-2 where the craton abuts on the Natal belt in the south. The change occurs within 30 km, suggesting that the anomaly largely reflects an increase in radioactive heat from the upper crust in the orogenic front. The change in mantle heat flux across the boundary is probably considerably smaller. These and other results suggest that the surficial craton boundary is further north than usually depicted. ¿ American Geophysical Union 1992