Two tiny species of aquatic ferns, free floating, not rooted to the bottom. The two Wisconsin species are distinguished from one another only by microscopic examination, and plants frequently lack the essential sori needed for identification. The species are listed below with no attempt to distinguish between them.

Azolla mexicana C. Presl, floating-fern is mostly restricted to the Mississippi River area in Wisconsin.
Azolla caroliniana Willdenow, Carolina mosquito-fern has been reported from a few locations in southern Wisconsin.