Unicellular small green algae. Flagellate or coccoid. The mamiellalean algae widely inhabit in maine, especially Micromonas, Batycoccuss and Osterococcus are important members of the phytoplankton community. Osterococcus is the smallest photosynthetic eukaryote (0.5 µm in diameter). The nuclear genome sequences of Micromonas and Osterococcus are reported.

Cells are naked or covered by a single layer of the spider-web scales. The squar / diamond small scales found in other primitive green algae are absent. Flagellate cell possesses one or two flagella inserted lateraly. The eyespot, if present, is located at the dorsal side of the cell. The mamiellalean algae usually possess prasinoxanthin as the major accessory photosynthetic pigment. Asexual reproduction by binary fission. Sexual reproduction is unknown.