As part of the U.S. Geological Survey's National Water-Quality Assessment Program, an investigation of the Yellowstone River Basin study unit is being conducted to document status and trends in surface- and ground-water quality. Surface-water samples are collected from streams or lakes at specific sampling stations. Water-quality characteristics at each station are influenced by the natural and cultural characteristics of the drainage area upstream from the sampling station. Efficient quantification of the drainage area characteristics requires a digital map of the drainage area boundary that may be processed, together with other digital thematic maps (such as geology or land use), in a geographic information system (GIS). Digital drainage-area data for 45 selected stream-sampling stations in the Yellowstone River Basin are included in this data release. The drainage divides were identified chiefly using 1:100,000-scale (50 m accuracy) hypsography. Drainage areas based on 1:100,000-scale hypsography data generally agree to within 5 percent with drainage areas measured at 1:24,000 scale, for areas larger than 50 km2.