hornwort order of flowering plants, consisting of just one family (Ceratophyllaceae) with one cosmopolitan genus (Ceratophyllum) that contains about six species.

Species of Ceratophyllum, called hornworts for their spiny fruits, are submerged aquatic plants that are mostly free-floating and occur in freshwater lakes and ponds and in slow-moving water. Ceratophyllum can rapidly choke waterways and is a breeding site for aquatic invertebrates and insects, including malaria-carrying mosquitoes (mosquito). Several species are used as aquarium plants.

Ceratophyllum lacks true roots and has dissected, whorled leaves with toothed margins. Flowers of Ceratophyllum have narrow and inconspicuous sepals or petals (these are sometimes interpreted instead as modified leaves or bracts), and there are separate staminate (male) and pistillate (female) flowers. Hornworts are unusual in having underwater pollination. When the male flowers are mature, the individual anthers break off and settle through the water until some reach a groove near the stigma of a female flower and lead to pollination.

Ceratophyllaceae was formerly thought to be the basalmost group of flowering plants, but that position is now attributed to Amborellaceae. Ceratophyllaceae is still considered to belong to the basal angiosperms (angiosperm) in the Angiosperm Phylogeny Group II (APG II) botanical classification system, but as an isolated lineage possibly sister to all the remaining flowering plants known as eudicots.