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The habitat, population density, distribution and diet of Octopus insularis Leite & Haimovici, 2008 were studied in the Atol das Rocas Biological Reserve, based on visual census carried out in fixed quadrants. For each octopus den, we collected data on octopus size, substrate, type of den, and food remains around it. A total of 100 octopus dens were found within the fixed quadrants. The highest density occurred at Mapas area (0.30 inds/100 m(2)) and the lowest at Crista Algalica (0.07 inds/100 m(2)). The preferred den type was the one on the reef bed (69%), reinforcing the importance of rocky substrates for this species. A total of 454 food items, classified into 22 distinct taxa, were collected. The class Crustacea was best represented, accounting for 70.5% of the total prey, followed by Gastropoda (22.4%), Bivalvia (6.4%) and Cephalopoda (0.7%). Five species represented more than 80% of all findings: the crabs, Xanthodius denticulatus White, 1848 (38.5%), Microphrys bicornutus (Latreille, 1825) (18.5%) and Mithrax forceps Milne-Edwards, 1875 (7.0%); and the gastropods Hipponix sp. (9.3%) and Collisella sp. (8.4%). The overwhelming preference for small crabs shows that O. insularis is a specialized predator. The strong presence of juveniles inside the ring reef indicates the importance of this location for the development of this octopus species in northeastern Brazil.