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    Background The oldest reference to marine life in Formigas Islets (oriental group of the Azores archipelago) goes back to the XVI century. Nevertheless, their macroalgal flora is poorly known, the published information mainly resulting from occasional collections of sporadic visitors. To overcome this and contribute to the knowledge of Azorean macroalgal flora at both local and regional scales, a thorough investigation was conducted in 1990 and 1991 under two expeditions promoted by the Marine Biology Research Group of the Department of Biology, University of the Azores. Collections and presence data recordings were done at the littoral and sublittoral levels down to approximately 40 m, in an area of approximately 0.04 Km2. This paper lists the taxonomic records and provides information regarding each species’ ecology and occurrence on the islets’ littoral. New information A total of 320 specimens are registered (including taxa identified only at generic level) belonging to 90 taxa of macroalgae, from which 70 were diagnosed at species level. The confirmed species comprise 39 Rhodophyta, 12 Chlorophyta and 19 Ochrophyta (Phaeophyceae), distributed by 22 orders (13 Rhodophyta, 3 Chlorophyta and 6 Ochrophyta) and 37 families (24 Rhodophyta, 6 Chlorophyta and 7 Ochrophyta). Sixty-one species represent new records for the islets, from which Botryocladia macaronesica Afonso-Carrillo, Sobrino, Tittley & Neto and Laurencia viridis Gil-Rodriguez & Haroun are Macaronesian endemisms. Most species are native to the Azores, but six have an uncertain origin and four are introduced (the Rhodophyta Asparagopsis armata Harvey; Laurencia dendroidea J.Agardh; Neoizziella divaricata (C.K.Tseng) S.-M.Lin, S.-Y.Yang & Huisman; and the Ochrophyta Hydroclathrus tilesii (Endlicher) Santiañez & M.J.Wynne). Introduction The Formigas Islets are located about 20 miles NE of Santa Maria Island and 34 miles SE of São Miguel Island (oriental group of Azorean archipelago, approximately 37°16′35″N, 24°46′54″W). They are arranged in a N-S direction, over a total length of about 165 m and a width of 80 m. Together with the submersed bank of Dollabarat, they form the Nature Reserve of Formigas Bank (DLR n° 11/88/ A). The oldest reference to life in Formigas Islets consists of descriptions of its marine fauna in the XVI century manuscript "Saudades da terra" written by the naturalist clergyman Gaspar Frutuoso. Subsequently, these islets were occasionally studied in sporadic visits by researchers, the first reference to the marine macroalgae being that of Piccone (1889). After that, several expeditions were made in order to study of the fauna and flora of the islets, which resulted in a few publications (see revision in Azevedo et al. 1991). An important finding was the first Azorean record of the brown alga Laminaria ochroleuca Bachelot de la Pylaie made by Ardré et al. (1973). Despite these efforts, the algal flora of these islets remained poorly known until the nineties, when a thorough investigation conducted by the Marine Biology Research Group of the Department of Biology, University of the Azores took place. This research group went to the islets in 1990 and 1991 and undertook collections and presence data recordings at the littoral and sublittoral levels down to approximately 40 m, in an area of approximately 0.04 Km2. Purpose This paper, aimed at contributing to a better understanding of the seaweed flora of the Azores archipelago, lists the macroalgae recorded on surveys undertaken at Formigas Islets (Azores, eastern group) and presents a general information for each taxon’s occurrence on the islets’ littoral, thus contributing to address several biodiversity shortfalls (see Hortal et al. 2015), namely the need to catalogue the Azorean macroalgae (Linnean shortfall) and improve the current information on their local and regional geographic distribution (Wallacean shortfall), as well as on species abundances and dynamics in space (Prestonian shortfall). It is intended as a resource for academics, students, government, private organizations, and the general public, and also as a practical basis for biological studies such as systematics, diversity and conservation, biological monitoring, climate change and ecology.