The Coober Pedy 1:250 000 sheet area (COOBER PEDY) is bounded by latitudes 29°S and 30°S and by longitudes 133°E and 135°E in the arid north of South Australia. The only town is the opal mining town of Coober Pedy on the northeastern margin of the sheet area which is readily accessible via the Stuart Highway. The area, generally, is well served with station tracks along the various vermin proof fences but accessibility is more difficult in the northwestern quarter where the Great Victoria Desert encroaches and tracks are much sparser. The new Tarcoola-Alice Springs railway will cross the sheet area approximately along the 134°30'E meridian. The bulk of the field mapping was done by V. Vitols, geologist, Geological Survey of South Australia during the winter months of 1973. This followed a number of spot observations (actually 70) made during the 1972 Department of Mines helicopter survey and during a ground reconnaissance a few months before that survey. Other geologists involved in the mapping during the 1972 operations were L.C. Barnes, G.W. Krieg, G.M. Pitt and A.F. Williams of the Geological Survey of South Australia. Geology was recorded on RC9 photography of the Department of Lands at a scale of approximately 1:80 000. Field checking and revision of, particularly, the Cainozoic is planned before publication of the Geological Sheet and Explanatory Notes.