Histories were recorded for 1030 American eider (Somateria mollisima dresseri) nests on 5 islands during 2 breeding seasons in Penobscot Bay, Maine. Nesting success was 39% in 1964 and 36% in 1965. Predation by gulls was responsible for most nest losses. Five factors were found to be related to nesting success: nesting cover, clutch size, partial predation, gull populations, and human disturbance. Nests in hardwood shrubs and cow parsnip (Heracleum maximum) had relatively high success while those in grasses and nightshade (Solanum dulcamara) were much less successful. Nests with a complete clutch of fewer than 4 eggs were less successful than those with 4 or more eggs. When part of a clutch was lost to predators the chance of ultimate success of the remaining eggs was reduced. The number of nesting gulls per nesting eider on a given island was inversely related to the nesting success of the eiders on that island. Human disturbance was indirectly responsible for lowered nesting success.