Cosmoledo is an atoll of the Aldabra Group and belongs to the Outer Islands of the Seychelles. The atoll is 14.5 km long east-west, and 11.5 km north-south. The total land area is about 5.2 km², while the lagoon measures 145 km² in area (total 152 km²). It is located at . The closest island is Astove Island, 38 km farther south. Cosmoledo Atoll and Astove Island are together sometimes known as Cosmoledo Group, which is part of the larger Aldabra Group.
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There are 16 individual islets and cays along the rim of the atoll, with Menai (in the west) and Wizard Island (Grande Île, in the southeast) being the largest. The islands, islets and cays are listed clockwise, starting with the northernmost island:
Cosmoledo holds Seychelles' largest colonies of all three species of booby that breed in Seychelles. The atoll holds the last viable population of Brown Booby, breeding mainly on Ile du Sud Ouest, with a few on Ile du Nord. It also has the Indian Ocean's larget population of Red-footed Booby censused by the Island Conservation Society as about 15,000 pairs. Exploitation has wiped out the Lesser Frigatebird and has almost done the same to the Great Frigatebird, which is down to just 10-20 pairs compared to hundreds of pairs in the 1970s. Despite this, hundreds of non-breeding frigatebirds of both species are to be seen, these being probably from Aldabra. The Lesser Frigatebird also once bred but now may be extinct. Cosmoledo also has Seychelles' largest colony of Sooty Tern breeding on the northern end of Grand Ile (Wizard). Second only to Aldabra are the numbers of breeding Red-tailed Tropicbird, Black-naped Tern and Crested Tern.
The lagoon is an important feeding ground for migratory waders, with Crab Plover and Ruddy Turnstone most common. There are three endemic races of land bird: the Madagascar White-eye (race menaiensi), the Souimanga Sunbird (race buchenorum) and a particular form of Madagascar Turtle Dove. The Madagascar Cisticola is also common and there are a few Pied Crow present. The most interesting islands are Grande Ile, Ile Sud Ouest, Ile du Nord, Ile du Nord Est and Ile du Trou.
The Island Conservation Society is carrying out conservation and rehabilitation work on the atoll, including the eradication of introduced rats and cats.