An inventory of the biological investigations conducted in the South Platte River Basin from 1891 to 1994 was done as a part of the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program in the South Platte River Basin. To aid in the sampling design of the biological component of the South Platte NAWQA, sources of water-related studies were compiled from computerized literature searches of biological data bases and by contacting other Federal, State, and local agencies. Biological investigations were categorized by their location in either of two major physiographic provinces-the Southern Rocky Mountains or the Great Plains, or in the transition zone between the mountains and the plains. From this collection of 102 references, five general categories of biological investigations were identified: algae, invertebrates, fish, habitat characterization, and chemicals in organism tissue. The most abundant literature was on studies of invertebrate and fish communities. Invertebrate studies primarily were conducted in the mountain region. There was limited information on algae, invertebrates in the plains region, flood-plain vegetation in the mountains and transition zone, and chemicals in organism tissue.