Plants small, free-floating, with hanging simple roots, without scales. Stems very slender and branching. Fronds alternate, 2-ranked, imbricate, 2-lobed, the upper lobe floating, green, with a hollow filled with mucilage and containing the threads of the cyanobacterium Anabaena azollae Strasb., the lower lobe thin, submerged and bearing the globose sori called sporocarps in pairs on cylindrical receptacles. Sporocarps consisting of either numerous microsporangia developed successively from the apex to the base, each consisting of 64 microspores, or of a single megasporangium containing a single megaspore. The microsporangia produce the mobile male gametes. These fertilise the megaspores in the larger sporocarps. After fertilisation the embryo develops within the wall of the megaspore until the first frond grows, when it emerges as a free-floating sporophyte.

Contains only one genus.

The presence of the nitrogen-fixing Anabaena has led to the use of Azolla in east Asia as a food for livestock and a green manure for rice fields. It is a practice which originated in China at least 2000 years ago.