The dasycladacean green alga Acetabularia antillana periodically appears in large numbers in shallow-water sheltered environments in the Florida Keys. The calcified aplanospores or reproductive cysts are known as calcispheres when fossilized. The hollow calcispheres from this alga are characteristically 140 to 185 mu m in dia. with a 10 to 25 mu m thick wall, and are composed of granular to acicular aragonite. The crystals are radiaxially oriented on the outer surface of the calcisphere and randomly oriented within the wall. A circular operculum 40 mu m diameter is present in intact calcispheres. When found in abundance calcispheres are excellent indicators of inshore well-sheltered environments. The heavily calcified growth form of A. antillana is approximately 74% skeletal carbonate by dry weight and produces up to 720 gm of aragonite per standing crop per m 2 of bottom area. This alga is an important local producer of aragonitic sediments in the less than 10 mu m size range.