The oldest reference to marine life in Formigas Islets (oriental group of the Azores archipelago) goes back to the 16th 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 undertaken 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.

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 in 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).