The euglenalean algae are basically unicellular flagellate, but Colacium forms a sessile non-motile stage attached to substrata (e.g. Daphnia). Most species are photosynthetic, but there are secondarily heterotrophic (osmotrophic) species such as "Astasia" and Khawkinea. Most euglenalean algae live in freshwater, but some marine species are also known. Euglena gracilis is widely used fro biological studies. The reserve polysaccharide of Euglena (paramylon) sometimes used commercially.

The cell is usually naked, but some species such as Strombomonas and Trachelomonas possess lorica deposited by iron and magnesium (Fig. 3). The cell possesses two flagella from the bottom of the anterior depresion, but one flagellum (no. 1) terminates in the depression. Many species show euglenoid movement, but others (e.g. Phacus, Lepocinclis, Monomorphina, Cryptoglena) show no metaboly. Secondarily heterotrophic species (e.g. "Astasia" longa) lose photosynthetic ability but retain plastid (leucoplast). Asexual reproduction by means of binary fission. Sexual reproduction is unknown.