A comprehensive water-quality investigation of the Yellowstone River Basin began in 1997, under the National Water-Quality Assessment (NAWQA) Program. Twenty-four sampling sites were selected for sampling of fish tissue and bed sediment during 1998. Organic compounds analyzed included organochlorine insecticides and their metabolites and total polychlorinated biphenyls (PCBs) from fish-tissue and bed-sediment samples, and semivolatile organic compounds from bed-sediment samples. A broad suite of trace elements was analyzed from both fish-tissue and bed-sediment samples, and a special study related to mercury also was conducted. Of the 12 organochlorine insecticides and metabolites detected in the fish-tissue samples, the most compounds per site were detected in samples from integrator sites which represent a mixture of land uses. The presence of DDT, and its metabolites DDD and DDE, in fish collected in the Yellowstone Park area likely reflects long-term residual effects from historical DDT-spraying programs for spruce budworm. Dieldrin, chlordane, and other organic compounds also were detected in the fish-tissue samples. The compound p, p'-DDE was detected at 71 percent of the sampling sites, more than any other compound. The concentrations of total DDT in fish samples were low, however, compared to concentrations from historical data from the study area, other NAWQA studies in the Rocky Mountains, and national baseline concentrations. Only 2 of the 27 organochlorine insecticides and metabolites and total PCBs analyzed in bed sediment were detected. Given that 12 of the compounds were detected in fish-tissue samples, fish appeared to be more sensitive indicators of contamination than bed sediment.Concentrations of some trace elements in fish and bed sediment were higher at sites in mineralized areas than at other sites. Concentrations of selenium in fish tissue from some sites were above background levels. Concentrations of arsenic, chromium, copper, and lead in some of the bed-sediment samples potentially exceeded criteria for the protection of aquatic life.