We studied methylmercury (MeHg) and total mercury (HgT) in impounded and natural surface waters in northwestern Minnesota, in settings ranging from agricultural to undeveloped. In a recently constructed (1995) permanent-pool impoundment, MeHg levels typically increased from inflow to outflow during 1997; this trend broke down from late 1998 to early 1999. MeHg levels in the outflow reached seasonal maxima in mid-summer (maximum of 1.0 ng L⁻¹ in July 1997) and late-winter (maximum of 6.6 ng L⁻¹ in February 1999), and are comparable to high levels observed in new hydroelectric reservoirs in Canada. Spring and autumn MeHg levels were typically about 0.1–0.2 ng L⁻¹. Overall, MeHg levels in both the inflow (a ditch that drains peatlands) and outflow were significantly higher than in three nearby reference natural lakes. Eleven older permanent-pool impoundments and six natural lakes in northwestern Minnesota were sampled five times. The impoundments typically had higher MeHg levels (0.071–8.36 ng L⁻¹) than natural lakes. Five of six lakes MeHg levels typical of uncontaminated lakes (0.014–1.04 ng L⁻¹) with highest levels in late winter, whereas a hypereutrophic lake had high levels (0.37–3.67 ng L⁻¹) with highest levels in mid-summer. Seven temporary-pool impoundments were sampled during summer high-flow events. Temporary-pool impoundments that retained water for about 10–15 days after innundation yielded pronounced increases in MeHg from inflow to outflow, in one case reaching 4.6 ng L⁻¹, which was about 2 ng L⁻¹ greater than the mean inflow concentration during the runoff event.