Little is known about the breeding biology of King Eiders (Somateria spectabilis), partly because they typically nest in remote areas, in low densities. The western North American population of King Eiders declined by more than 50% between 1976 and 1996 for unknown reasons (Suydam et al. 2000). Additionally, the National Petroleum Reserve-Alaska (NPR-A) is being leased for oil and gas exploration and may potentially be developed. The highest known density of nesting King Eiders on the north slope of Alaska is within the northeast planning areas of NPR-A (Larned et al. 2003). During the summers of 2002 and 2003, we studied King Eiders in an area to the southeast of Teshekpuk Lake within the NPR-A, and in the Kuparuk oilfields on the North Slope of Alaska to provide information on their basic breeding biology and habitat use. We compared timing of nesting, nest success, and habitat use between a relatively undisturbed site at Teshekpuk Lake and the active Kuparuk oilfield.