Water-quality investigations in six interconnected lakes that comprise most of the surface area of Voyageurs National Park in northern Minnesota revealed substantial differences in water-quality. Three large lakes; Sand Point, Namakan, and Rainy, near the eastern and northern boundaries of the Park; are oligotrophic to mesotrophic, having low dissolved solids and alkalinity, and dimictic circulation. In contrast, Kabetogama Lake, Black Bay, and Sullivan Bay, near the western and southern boundaries of the Park, were eutrophic, having higher dissolved solids and alkalinity, and polymictic circulation. Chemical characteristics of the three lakes along the eastern and northern boundary were similar to those of the Namakan River--a major source of inflow that drains an extensive area of exposed bedrock and thin noncalcareous drift east of the Park. The lake and embayments along the western and southern boundary receive inflow from two streams that drain an area west and south of the Park that is overlain by calcareous drift. Samples from one of these streams contained dissolved-solids concentrations about five times, and total alkalinity concentrations about eight times concentrations measured in the Namakan River. The nutrient-enriched lakes and embayments had high algal productivity that produced blooms of blue-green algae in some years. Annual patterns in the levels of trophic-state indicators revealed that the shallow, polymictic lakes experienced seasonal increases in totalphosphorus concentrations in their euphotic zones that did not occur in the deeper, dimictic lakes; this indicates a link between the frequent recirculation of these lakes and internal cycling of phosphorus. Secchi-disk transparency was limited by organic color in Sand Point, Namakan, and Rainy Lakes, and resuspended bottom material reduced transparency in Black Bay. Waters in the large lakes and embayments met nearly all U.S. Environmental Protection Agency criteria for protection of freshwater aquatic life, recreation, and drinking water. Some sites exceeded criteria because of oil and grease, phenols, sulfide, and ammonia. Reconnaissance sampling of 19 small lakes in remote areas of the Park indicated that most of them are sharply stratified and had very low dissolved solids and alkalinity concentrations (4.0-29 milligrams per liter total alkalinity). Thirteen of the lakes could be classified as moderately sensitive to acid precipitation, and two could be classified extremely sensitive. About half of the interior lakes had low nutrient concentrations (10-30 micrograms per liter total phosphorus) and low algal productivity (0.1- 2.0 micrograms per liter chlorophyll a). Five of the lakes had a marked reduction in trophic state from spring to summer. The Namakan River is the largest source of inflow to the Park and was found to have better quality than its receiving waters based on dissolved solids and nutrient concentrations, algal productivity, and transparency. The Ash River was found to deliver water that generally was poorer in quality than its receiving waters.