Euglena is a well-known genus of flagellate protozoa, typical of the euglenids, and commonly found in nutrient-rich freshwater, with a few marine species. The cells vary in length from around 20 to 300 μm, and are typically cylindrical, oval, or spindle-shaped, with many bright green chloroplasts and a single emergent flagellum. Euglena was first named by Ehrenberg in 1830, and since then over 100 species have been described. The name comes from the Greek words eu and glÍne, or good eye, referring to the light-sensitive eyespot.
Genetic studies suggest that the genus Euglena is paraphyletic to various other uniflagellate euglenids. Most species fall into two major lineages: those which exhibit metaboly, including colourless species normally treated as Astasia and Khawkinea, and those with more rigid pellicles, including some normally included in Lepocinclis.