Most are free-living in freshwater, such as the commonly studied genus Tetrahymena, but some are parasitic on fish or aquatic invertebrates.

[1] Among these is the important species Ichthyophthirius multifiliis, a common cause of death in aquaria and fish farms.

The oral cilia are in general distinctly tetrahymenal, with three membranelles and a paroral membrane, which corresponds only to the middle segment of the tripartite membranes found in certain scuticociliates.

Mouth formation during cell division usually begins next to a postoral kinety.

Initially the scuticociliates and peniculids were included, then later treated as separate orders of a subclass Hymenostomatia, to which the astomes are sometimes added.