Fontinalis antipyretica is always found in aquatic sites. It may be found in moving water attached to rocks and logs, or floating freely in stagnant water.
Fontinalis antipyretica is a large pleurocarpous moss whose dark green, floating or reclining shoots can sometimes exceed a metre in length!
The leaves on a shoot are three-ranked, keeled, and straight. The stem is often red.
The keeled leaves lack a costa (ecostate).
Sporophytes are rare, however may occur in autumn or spring.
The mature more or less immersed sporangium is surrounded by leaves and may remain submerged. Spore dispersal in an aquatic environment probably obviates the need for a long seta.
A mesh-like structure in the shape of an inverted cone called a trellis is characteristic of the Fontinalis endostome.