The accompanying contour map was prepared by using a Cretaceous structural datum which covers most of the Great Artesian Basin in South Australia. The present edition of the map includes information available to January 1968 and represents a second revision of a contour map prepared by the Petroleum Exploration Section in March 1963 (Freytag 1963). At that time datum control was limited to water bores concentrated near the Basin margin, a dozen or so exploration wells, and to widely separated reconnaissance seismic reflection traverses A few anticlinal structures were covered by semi-detailed seismic survey, but large areas of the Artesian Basin in South Australia were completely unexplored in the subsurface. During the following 2 years a steady increase in Petroleum exploration resulted in an influx of new data. Seismic surveys conducted by private contractors and the Mines- Department now cover all but small areas of the Basin and provide extensive direct datum control for structural evaluation. Widespread gravimetric surveys show intrinsic structural features of the Basin. The stratigraphic have been clarified by further exploratory wells. Field reconnaissance by the Petroleum Exploration Section has indicated important lateral variations in the datum formation used for this map, and Departmental mapping in the Marree and Oodnadatta areas has added to our structural and stratigraphic knowledge in the zones of marginal crop.