The island fauna has special characteristics that depend not only on biotope diversity, but also on the island’s geographical features: area, provenance, distance from the mainland, and hydrodynamic processes. Black Sea Biosphere Reserve includes islands located in Tendrivska and Yagorlytska bays. The islands of Babyn, Smalenyi, Potiyivski, Sybirski, and others are located in Tendrivska Bay. The island of Tendrivska Kosa (Tendra Island) separates Tendrivska Bay from the Black sea. In Yagorlytska Bay, there are the Malyi and Velykyi Kinskyi islands, Dovhyi and Kruhlyi islands, and the Yehypetski Islands. The total area of the islands of protected bays of the Black Sea Biosphere Reserve is 3365 hectares, and the length of the outer coastlines is 179.5 km. The following biotopes were identified on the islands: reed thickets, sandy and shell beaches on the sea coasts and coasts of the deep-water parts of the bays; flooded solonchaks; the inner parts of the islands with plant associations that are typical for the coastal steppe and the littoral-coastal complex. The mammal assemblage of the islands includes 18 spe¬cies belonging to 6 orders, 10 families, and 16 genera. From 0 to 15 species of mammals live on individual islands. The species composition of the mammal fauna depends on the area of the island, available biotopes, the possibility of animal exchange between the island and the mainland. A number of islands have no other inhabitants except birds during the nesting period, primarily colonial ones. A number of islands belong to the complex of reed thickets, in particular the Yehypetski and Sybirski islands. These islands are located along the mainland coast and are regularly visited by wild boars and carnivorans; other islands are littoral (Novi, Potiyivski), and there are no permanent residents on them. Other islands are covered only with reeds and have no permanent inhabitants. On large islands, such as Dovhyi and Tendrivska Kosa, there are 11–15 species of mammals (migrating dolphins and bats not included), of which almost 20% are invasive and associated with human activity. The species composition and state of populations of micro- and mesomammals on the islands depend on many factors, mainly catastrophic flooding, storms, local epizootics, which can occur due to both natural abiotic and anthropogenic factors.