Abstract. – The paper deals with the composition of the arthropod fauna in nineteen shag (Phalacrocorax aristoteli Linnaeus, 1761) nests collected in two sampling sites from the National Park of the Atlantic Islands: in the Cíes and Ons Islands located in the NW of the Iberian Peninsula during 1997. The occurrence and abundance of arthropods in the nests have been studied in three different breeding phases: nests with eggs, nests with broods and empty nests. In these nineteen nests, the authors found 25 families of acari (92.1% of the total fauna), 14 families of Diptera (98% were in a saprophagous larval stage), 8 families of Hymenoptera, 16 families of Coleoptera and 5 families of spiders. The ecological and the trophic relations of these families have been studied. A large proportion of arthropod families found in the nests occur there occasionally. Furthermore, the nests provided few groups with the right conditions to develop and complete their biological cycle. Only saprophagous mites, fly larvae and predator beetles found suitable conditions to survive in the nests. Saprophagous arthropods were the most abundant, independently of the nest occupation phase. The difficult access to the cliffs along with the necessity to transport the nests into the laboratory hampered the sampling process and consequently made this study the first about entomological fauna in shag nests.