An account is given of the blue-green algal and some other communities in the intertidal region of the lagoon at Aldabra Atoll. Data are included on environmental parameters and on standing crop as indicated by chlorophyll a. A more detailed study on one species, Scytonema sp., includes observations on recolonization over a six-month period. Visually obvious blue-green algal communities cover about 19% of the intertidal region of the lagoon at Aldabra, being second only to mangroves in importance. Other taxa are much less abundant: an approximate estimate for eukaryotic algae indicated that their cover is between one and two orders of magnitude less, while that of marine angiosperms and phototrophic bacteria is still less. The two most abundant species of blue-green alga are Scytonema sp. and Microcoleus chthonoplastes, with the former in the upper zone of the littoral and often also the mid zone, and the latter in the mid zone of most parts of the lagoon and almost everywhere in the lower zone. M. chthonoplastes is most conspicuous along the southern shore, west of Bras Anse du Bois, where it forms an irregular, but almost complete cover over extensive areas of white silt, with the community often showing conspicuous laminations. The most obvious competitors to these two species are Rivularia sp. and Schizothrix calcicola, respectively. An extensive cover of well developed stromatolites occurs only in the southwest part of the lagoon, a region relatively well sheltered and with a tendency for waters to become moderately hypersaline. In addition to epiliths, endolithic blue-green algae provide an almost complete cover on all intertidal rocks, and recolonize a broken rock surface rapidly, both features showing a marked contrast to terrestrial rocks. It is suggested that grazing animals favour the success of endoliths over epiliths and that it is the synergistic combination of endolithic blue-green algae and grazers which is responsible for biological erosion. The endolith Mastigocoleus testarum is exceptional among all the lagoon algae in being the only heterocystous species to occur in the lower zone. Communities on the open lagoon shore of Ile Picard differ in several ways from those elsewhere; Scytonema sp. is more closely restricted to the upper zone; the mid zone is dominated by Hyella balani endolithic in sand particles; Chromatium is sometimes conspicuous. It seems probable that these phenomena are all due to a predominantly horizontal drainage of mangrove forest water displaced at spring tides, through sediments that are much coarser here than elsewhere on the atoll. Away from Ile Picard, visually obvious phototrophic bacteria are mostly restricted to mangrove forests, usually at the sides of creeks, though at Cinq Cases they are sometimes abundant in shallow pools.