During September 2006, samples from public water-delivery systems, ground water, surface water, bed-sediment, and fish-tissue sources were collected at 32 locations within the Standing Rock Sioux Reservation and analyzed to aid in the evaluation of any immediate water-quality concerns. Samples were collected from Solen, Selfridge, and Fort Yates, North Dakota, water-delivery systems and included raw water samples and treated water samples from water users on the water-delivery systems. Samples from the Solen and Selfridge systems were analyzed for dissolved major inorganic and dissolved minor and trace inorganic concentrations. Samples from the Fort Yates system were analyzed for dissolved major inorganic concentrations, dissolved minor and trace inorganic concentrations, total and dissolved nutrient concentrations, total and dissolved organic carbon concentrations, and volatile organic compound concentrations. Water samples were collected from ground-water wells throughout the reservation only in North Dakota and analyzed for dissolved major inorganic concentrations and dissolved minor and trace inorganic concentrations. Water samples were collected at locations on the Missouri River and its major tributaries within the reservation and analyzed for dissolved major inorganic concentrations and dissolved minor and trace inorganic concentrations; bed sediment was collected at these sites and analyzed for minor and trace inorganic concentrations. Fish-tissue and liver samples were collected from several species on the Missouri River near Fort Yates and analyzed for minor and trace inorganic concentrations. Results of the water-quality analysis revealed very little of concern, with the exception of elevated boron concentrations in the drinking water and ground water in the area of Selfridge and Solen and minor exceedences of total trihalomethanes in the Fort Yates water-delivery system.