Harlequin Ducks (Histrionicus histrionicus) and aquatic insects have been monitored on the River Laxá at Myvatn, from 1975 and 1977, respectively, up to the present. The area holds the densest breeding population of this species in Iceland. The relationship between food abundance, reproductive success and density of breeding Harlequin Ducks were examined on three sections (outlet, upper and lowland) of the Laxá. Spring density was highest on the lake outlet (2.1 ha^-1) and much lower on the upper (0.3 ha^-1) and lowland (0.4 ha^-1) sections. Numbers of Harlequin Ducks on the Laxá increased significantly during the study period, 1975-2002. The trend of increasing numbers could be attributed to either a general, as yet unexplained, increase of Harlequin Ducks in northern and northeastern Iceland, or perhaps to the successful eradication of American Mink (Mustela vison) breeding at the lake outlet (the main production centre of young Harlequin Ducks). Sex ratio in spring was 41% females at the outlet and on the upper Laxá but 36% on the lowland part. Yearling males were about 1% of all males on the upper river but 6-19% on the lowland part. Most young were produced at the outlet, while few females or broods were found further down the river. The mean total number of young produced annually at the outlet was 74 ± 11 (range = 3-235). Young per female present in spring at the outlet and upper part was 0.49 ± 0.09, or if only females at the outlet in spring are considered, 0.82 ± 0.17. Annual production of young was positively correlated with total yearly black fly abundance. We conclude that food limits the production of young in the Harlequin Duck. Changes in the densities of adults on the breeding grounds showed density dependence but were not significantly associated with food resources or previous production of young.