Hapaloaiphon intricatus (Cyanophyceae) with lightly calcified sheaths, is the dominant filamentous alga reported from a humid, dimly-lighted limestone cave in northcentral Florida (USA). Within the limestone substrate, hydroxyapatite is the form of the phosphate mineral providing nutrient to the algae.The Crystal River geological formation of the Ocala Group [12] in northcentral Florida exposes in some locations a surface area rich in caves. These are composed of white, nearly pure limestone with varying quantities of calcium and magnesium carbonate. Small amounts or traces of phosphates are not uncommon in the rocks of these caves [13]. Although the fauna of the Florida caves has been the subject of a number of reports [5,14], the algal flora has received but scant attention. To the best of our knowledge, no studies concerned with phosphate sources for cave plants are available. The present paper reports the occurrence of the dominant calcifying filamentous blue-green algae growing in Orange Lake cave, and the mineral species in the substrate supplying phosphates to the plants. Orange Lake cave is located 0.4 miles south of junction of US Hwy 441 and State route 318 offHwy 441 (T.12S, R.21E, See. 33/34), in Marion County, Florida. The highly fossiliferous cave of limestone was developed in the Crystal River Formation during the late Eocene [5]. The cave opens on the north side of an abandoned dry limestone quarry of several hundred hectares. Two openings (2 • 1.5 m; 1.5 • 3 m) lead to a small, dimly-lighted room; a lateral passage then leads to a dark room housing a bat population. Ferns, fern prothallia, mosses, liverworts and lichens grow abundantly from the mouths to 1.0 m toward the inside of the cave. In the dimly lighted room, extensive bluegreen areas of coccoid blue-green algae on the walls and roof contrasted with scattered red-brown patches on the roof whose sizes ranged from a few mm 2 to 10-20 cmL With a hand lens, the red-brown patches resembled tightly matted tufts of hair 0.25-0.3 mm in height. When brushed with a firm object, the patches were soft and yielding. Light intensity in this area (2-3 m from the ,) Florida Agricultural Experiment Station Journal seres No. 4381.