This extraordinary aquatic fern (plus its symbiotic, nitrogen-fixing, blue-green alga) is widely scattered over southeastern states, but generally restricted to old, stable, stagnant water-bodies with moderately eutrophic conditions. The closely related western species, A. mexicana Presl., is known in adjacent Mo. (FNA 2; Y), and is expected in far western counties of Ky. It differs from caroliniana in the larger size of its fronds (ca. 10-15 mm wide versus 5-10 mm), and in its glochidia (appendages of microspore packets) with several cross-walls (versus none).