Microatolls of coralline algal encrustations were discovered during a reconnaissance study of environments around Cozumel Island, 14 mi off the NE. coast of the Yucatan Peninsula. In one small area along the northeastern shore 166 of them were counted in a belt approximately 100 ft wide, 150 yd offshore, and parallel to shore. The microatolls are roughly circular with diameters of 12-25 ft. The outer margin of each is outlined by an algal ridge several inches wide and a few inches above sea level. This rim, enclosing a shallow lagoon, is especially because it supports a luxuriant growth prominent of upright, noncalcareous algae. Many of the microatolls are within 15 ft of one another; the deepest passages observed between them are 14 ft deep. The microatolls form the upper rims of cylindrical structures with vertical to slightly overhanging sides. The sides are covered by coralline algae including Goniolithon solubile, Archaeolithothamnium episporum, Lithothamnium sejunctum, Epilithon membranaceum, Lithophyllum sp., and Lithoporella sp. Living coral colonies cover only a small percent of the vertical sides, but most samples show coral beneath the algal encrustation. The origin of the microatoll foundations is speculative.