Methylmercury can impair the reproduction of fish-eating wildlife. We measured lake pH, mercury (Hg) concentrations in small fish, blood Hg levels in adult male, female and juvenile common loons, and loon productivity from 120 lakes in Wisconsin, USA and New Brunswick and Nova Scotia, Canada (Maritimes). Mean lake pH was higher in Wisconsin than in the Maritimes. Body masses of adult loons and Hg concentrations in the blood of loons and in small fish were greater in the Maritimes. Hg levels in fish increased with lake acidity. Abundance of small fish increased in acidic lakes in the Maritimes. Blood Hg concentrations in adult and juvenile loons decreased with lake pH and increased with Hg levels in fish prey. Hg levels in male, female and juvenile loons were 22, 16 and 2.3 times greater than those in small fish. Loon Hg exposure, measured either as Hg levels in female loon blood or in fish prey, appeared to impose an upper limit on loon productivity. Loon productivity decreased as Hg exposure increased. Quantile regression analysis indicated that maximum observed loon productivity dropped 50% when fish Hg levels were 0.21 ug/g (wet wt), and failed completely when fish Hg concentrations were 0.41 ug/g.