Alacran "Reef" is an oval, shallow-water platform approximately 100 sq. mi. in extent which rises above the general surface of the continental shelf 70 mi. off the Yucatan Peninsula of Mexico. The reef complex includes the long, arcuate reef which forms the windward side of the platform, a deeper and less sharply defined belt of reef growth outlining the leeward margin, and a multitude of reefs of various shapes and sizes in the enclosed lagoon. Contemporary sedimentation at Alacran is resulting in 3 major types of accumulations: loose sediment, rigid frames, and nonrigid frames. The loose sediment, primarily skeletal debris in the form of silt, sand, and gravel, covers inter-reef parts of the lagoon bottom. The rigid frames are built by a succession of organic overgrowths during which new colonies spread over the in situ skeletons of their predecessors. Nonrigid frames are being built on the protected side of some rigid frames where branching colonies of coral or algae are so abundant on the loose sediment surface that the individual colonies nearly touch each other, or even intermesh, although they are not cemented to one another. In terms of their roles as geologic agents, the organisms of the area can be categorized as frame builders, cementing agents, sediment contributors, frame destroyers, inhibitors of reef growth, and sediment movers. Many nonfossilizable members of the fauna and flora have considerable influence on sediment character and on rate and place of sedimentation. Lateral changes in benthonic organisms and bottom character are prominent on and around the Alacran reefs. Reef surfaces are divisible into 2 and sometimes 3 zones based on distribution of species which build rigid frames. Two other zones are based on distribution of species which build nonrigid frames. Other zones, such as one recognizing prominence of sediment-binding plants, are differentiated in lagoonal areas between reefs. All the Alacran reefs appear to be thriving, or at least holding their own in relation to the destructive forces of wave action and burrowing organisms. The outward expansion of the reefs into deeper water seems to depend primarily on some of the massive corals which attain great size but do not require a hard substrate. Such species create the foundation of reef rock on which other frame builders can establish and to which they subsequently contribute. Several environmental factors influence the morphology of individual coral colonies. Change in colony form correlated with increase in water depth is well shown by one species of branching coral, and several other species illustrate the influence of wave action, sedimentation rate, and the presence of intruding organisms on colony morphology. The reefs which are exposed to the strongest wave action differ in several respects from those in more protected areas. Some of the major frame-building coral species on less protected reefs are absent or uncommon on most lagoonal reefs. Encrusting calcareous algae contribute to windward reef frames but are inconspicuous in lagoonal reefs. Study of reef surfaces suggests that cross sections of lagoonal reefs would show less tightly knit reef frames with larger pockets of loose sediment than would be the case in windward reefs.