The specific habitat requirements of harlequin ducks (Histrionicus histrionicus Linnaeus 1758), which breed in the Pacific Northwest, are largely unknown. I captured and attached radio transmitters to 42 female harlequins (36 paired and 6 unpaired) in the central Cascade Range of Oregon in 1994 and 1995. Breeding pairs used 3rd through 5th order streams but nesting occurred near 1st through 5th order streams. Males returned to the coastal habitats at the end of May, but females remained on streams during the summer; non- and failed-nesting females departed in late July through mid-August. Successful hens and young departed in late August. Nests were located for 20 of the 36 paired females; 15 of 20 nests were successful. An additional successful nest (of an unmarked hen) was located during the fieldwork. Ninety-five percent of nests had overhead cover within 0.75m. Nests were in a variety of forest types ranging in age from 30-300 years old. Seven nests were located in the floodplain; 5 were on islands and 2 were on streambanks. Four of these nest were on perched on wood debris. Thirteen nests were above the floodplain; 9 on terraces or cliffs and 4 on side slopes and all were on the ground. Clutches averaged 5.5 eggs with 74% apparent nest success. Duckling survival was 60%, while brood density on streams ranged from 0.15-1.2 brood/km. First to 5th order streams and riparian buffer zones with diverse habitat types are important when considering harlequin duck management options.