The pelagophyceaean algae are unicellular, sarcinoid, palmelloid or filamentous. Most species are non-motile but some species are flagellate. The Pelagophyceae includes many picoplanktonic species (Pelagomonas, Aureococcus, Aureoumbra). Photoautotrophic. Marine. Aureococcus and Aureoumbra sometimes fom large-scale blooms (brown tides).

The non-motile cells are usually covered by cell wall, mucilage or theca. The flagellate cells covered by thin theca possess two (rarely one) flagella. The transitional region of flagella includes no transitional helux but proximal helix. No stigma. The yellowish chloroplasts include fucoxanthin, 19'-butanoyloxyfucoxanthin, diadinoxanthin and diatoxanthin. Asexual reproducion by means of binary fission and zoosporogenesis. Sexual reproduction is unknown.