The Big Sioux River basin of eastern South Dakota contains an important surface water supply and a sizeable aquifer system of major importance to the economy of South Dakota. The aquifers are complex, consisting of many small aquifers that are hydrologically associated with several large aquifers and the Big Sioux River. The complexity and interrelation of the surface water/groundwater systems has already created management problems. As development continues and increases, the problems will increase in number and complexity. To aid in planning for future development, an accurate determination of drainage areas for all basins, sub-basins, and noncontributing areas in the Big Sioux River basin is needed. All named stream basins, and all unnamed basins > 10 sq mi within the Big Sioux River basin in South Dakota are shown and are listed by stream name. Stream drainage basins in South Dakota were delineated by visual interpretation of contour information shown on U.S. Geological Survey 77-1/2 minute topographic maps. One table lists the drainage areas of major drainage basins in the Big Sioux River basin that do not have a total drainage area value > 10 sq mi. Another shows the drainage area above stream gaging stations in the Big Sioux River basin. (Lantz-PTT)