Extensive laminated mats of algae are forming on the protected intertidal and supratidal flats of a highly saline lagoon, the Khor al Bazam, Abu Dhabi, southwest Persian Gulf. At the east end of the lagoon, the largest algal flat parallels the coast for 42 kilometers, and to the west another smaller one parallels the coast for nine kilometers. These flats, part of the seaward edge of a prograding coastal flat, have an average width of approximately two kilometers and a thickness of at least thirty centimeters. In some areas they extend landward in the subsurface for more than two kilometers, beneath a thin cover of evaporites and wind blown and storm washover sediments. Smaller flats occur in the shelter of islands, headlands, and swash bars. The larger algal flats are divided on the basis of surface morphology, into four geographical belts. From the high-water mark seaward these are: (1) Flat zone--firm, smooth algal mat, with no topographic relief, overlying quartz-rich carbonate sand and evaporites; (2) Crinkle zone--leathery algal skin forming a blistered surface over gypsum and carbonate mush; (3) Polygonal zone--algal mat separated into desiccation polygons a few centimeters to two meters in diameter, which cover laminated algal peat; carbonate sand and mud fills the cracks between the polygons. (4) Cinder zone--a warty black algal surface, the color and size of the raised bumps resembling a weathered volcanic cinder layer. These bumps, shaped like small pustules two to three centimeters in diameter, cap an unlaminated algal and sediment peat. The algal growth and structures appear to be determined by the frequency and duration of subaerial exposure and the salinity of the tidal waters; they are only related to wave energy when limited by wave and tidal scour at the edge of the Cinder zone and along ebb channels.