Abstract  We studied methylmercury (MeHg) and totalmercury (Hg_T) in impounded and natural surface waters innorthwestern Minnesota, in settings ranging from agriculturalto undeveloped. In a recently constructed (1995) permanent-pool impoundment, MeHg levels typically increased from inflowto outflow during 1997; this trend broke down from late 1998 toearly 1999. MeHg levels in the outflow reached seasonal maximain mid-summer (maximum of 1.0 ng L^-1 in July 1997) andlate-winter (maximum of 6.6 ng L^-1 in February 1999), andare comparable to high levels observed in new hydroelectricreservoirs in Canada. Spring and autumn MeHg levels weretypically about 0.1–0.2 ng L^-1. Overall, MeHg levels inboth the inflow (a ditch that drains peatlands) and outflowwere significantly higher than in three nearby referencenatural lakes. Eleven older permanent-pool impoundments andsix natural lakes in northwestern Minnesota were sampled fivetimes. The impoundments typically had higher MeHg levels(0.071–8.36 ng L^-1) than natural lakes. Five of six lakesMeHg levels typical of uncontaminated lakes (0.014–1.04 ngL^-1) with highest levels in late winter, whereas ahypereutrophic lake had high levels (0.37–3.67 ng L^-1)with highest levels in mid-summer. Seven temporary-poolimpoundments were sampled during summer high-flow events. Temporary-pool impoundments that retained water for about 10–15days after innundation yielded pronounced increases in MeHgfrom inflow to outflow, in one case reaching 4.6 ng L^-1,which was about 2 ng L^-1 greater than the mean inflowconcentration during the runoff event.