In Iceland, the Harlequin Duck (Histrionicus histrionicus) is generally distributed as a breeder on rapid clear-water streams, from sea level to about 700 m. Densities are generally low, except at the headwaters of streams draining productive lakes where blackflies (Simulium vittatum) are abundant. In northwest, north and east Iceland, molting Harlequin Ducks are found in similar localities as the wintering birds, but molting birds are largely absent from the southwest coast. During the molt, the birds often stay close to cliffs or rocks and are hard to find. Harlequin Ducks winter on exposed rocky coasts and densities are correlated with substrate as indicated by shore exposure. In 1998-2001, the total wintering population of Harlequin Ducks in Iceland was estimated at 14,000, with 95% confidence limits of 12,000 to 16,000. Assuming that migration in and out of Iceland is insignificant, the total Icelandic breeding population is 3,000-5,000 females, or three to five females (pairs) 100 km^-2. A decline in wintering numbers was found in southwest Iceland in 1961-2002, but numbers in the northeast have increased. This may be related to increased temperatures, but changes in exploitation may also have affected the population. The conservation of the Icelandic population of the Harlequin Duck is largely a matter of wise use of the coast and streams which should include preemptive measures, such as minimizing pollution and development. Small-scale river development for power production and salmon fishing pose potential threats to Harlequin Ducks, but direct exploitation of this population is not likely.