Plants aquatic, floating on placid water, occasionally stranded, subsisting on mud; plants heterosporous (producing 2 kinds of spores), leptosporangiate, proliferous by axillary fragmentation. Roots translucent to brown, lax, singular [in bundles] without branches, emerging at stem branch points; root hairs to 1 cm, emerging from root cap. Stems usually not green, with extensive subdichotomous branching, prostrate and reniform or polyreniform, or nearly erect. Leaves sessile, alternate, often imbricate, in 2 ranks along upper side of stem, 0.6–2 mm wide; each leaf with 2 lobes; upper (emersed) lobe greenish or reddish and photosynthetic, with narrow colorless margin, several cells thick, bearing colony of blue-green algae (Anabaena) in ovoid cavity at base of lower side; lower lobe often floating or immersed, slightly larger than upper lobe, mostly not green (often colorless and translucent), 1 cell thick except at base, ± cup-shaped. Sporocarps in pairs [tetrads] at base of lateral branches, members of pair of same sex or of different sexes. Megasporocarps containing 1 megasporangium that produces 1 functional megaspore. Megaspore spheric, 0.2–0.6 mm, topped with dark, conic, slightly narrower structure (indusium) covering 3  floats and a blue-green algal colony. Microsporocarps globose, apically umbonate, 10–27µm diam., containing to 130 microsporangia; microsporangia containing 32 or 64 microspores 3 µm diam., aggregated into 3–10 masses covered with arrowlike barbs [glabrous or with needlelike hairs on 1 side].
Worldwide, tropical to temperate regions.
Azollaceae has been included in Salviniaceae, but the relationship is not close.
In this treatment, "upper lobe" refers to the emersed lobe and "lower lobe" refers to the immersed lobe. Developmentally, the emersed lobe is actually abaxial and the immersed lobe is adaxial. To facilitate identification in the field, however, the terms describe the appearance of the lobes to the viewer, not the development of the lobes.
Genus 1, species ca. 7 (3 species in the flora).