Microscopic filamentous, pseudofilamentous or unicellular red algae. Most species live in coastal water, but Rufusia is reported from the hairs of of three-toed sloths.

The shape of chloroplast is variable and color of chloroplast is reddish to blue-greenish. Most species possess peripheral thylakoid. The low molecular weight carbohydrates are sorbitol (Stylonematales) or floridoside (Rufusia). Golgi body is associated with ER and miochondrion. Pit connection is absent. Asexual reproduction by binary fission or monospores. Sexual reproduction is unknown.