The East and West Flower Garden Banks in the northwestern Gulf of Mexico represent the northernmost tropical coral reefs and Caribbean coralline algal flora on the North American continental shelf. Coralline algae are the dominant framework builders and sediment contributors below 50 m on the banks. On the basis of growth form and taxonomic composition, coralline algae on the Flower Garden Banks can be divided into four overlapping depth zones: upper coral reefs (20-30 m), Madracis -algal ridges (30-40 m), algal nodule zone (45-80 m), and deep algal reefs (50-90 m). Corallines on the upper coral reefs are dominated by Hydrolithon, Lithophyllum , and Porolithon . Their growth form as thick, laterally restricted crusts and knobs results from intensive grazing by parrotfish and urchins. Grazing is diminished on the Madracis -algal ridges below the upper reefs, and corallines grow here as laterally extensive plates, binding coral rubble. Dominant corallines on the ridges are Lithophyllum, Paragoniolithon , and Tenarea . Algal nodules (rhodoliths) on the deeper bank slopes are the most widespread algal growth form on the Flower Garden Banks. On the basis of shape, internal structure, and composition, rhodoliths are divided into an upper zone (where they have a spherical shape with concentric laminae) and a lower zone (discoidal shape with irregular laminae). Diminishing current velocity with increasing depth controls nodule morphology. The red alga, Peyssonnelia , and the foraminifer, Gypsina , are two non-coralline components that contribute substantially to nodule growth. Reduced current velocity in the lower nodule zone enables corallines and Peyssonnelia to develop laterally extensive pavements and 1-2 m high algal reefs. Taxonomic composition, high framework porosity, and irregular internal growth morphology of the deep algal reefs distinguish these structures from shallow algal ridges described elsewhere. The 90 m isobath at the Flower Garden Banks approximates the upward limit of a turbid nepheloid layer composed of continually resuspended sediment. Reefal structures within this water layer are dead and are now covered by fine sediment.