Bird nests that persist for many years are frequently inhabited by invertebrates such as fleas, lice, ticks and mites which can negatively affect the fitness of the incubating birds. Little is still known of how densities and species composition of the parasites relates to habitat selection or habitat use of the birds, and consequently their means to lower these negative effects. The invertebrate fauna of nests (down and bottom material) of the common eider (Somateria mollissima) was compared between two eider colonies in Breidafjordur, West Iceland, i.e. in one sparsely nested offshore colony (Hvallátur) and in one densely nested coastal colony (Rif). The invertebrate communities at both sites had low species diversity and were dominated at both sites by the flea Ceratophyllus garei, which was observed in all nests investigated at high densities. The tick Ixodus uria was also found at both sites but in much lesser abundance. Higher densities of fleas and mites were seen at the nests at the sparse offshore eider colony at Hvallátur, than in the dense coastal colony in Rif. Other organisms found were non-parasitic insects and soil mites (Acari: Oribatida) which were probably associated withthe bottom material in the nest bowls or the soil. There was no relationship between species or abundance of fleas and the nest materials (red fescue grass, marram grass and dry brown seaweed). These differences are probably mostly related to the behavior of the females, rather than to the density of the colony or the offshore/inshore location of the colony.