Euglena is a genus of single-celled flagellate protists. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 800 species. Species of Euglena are found in fresh and salt waters. They are often abundant in quiet, inland waters, where they may bloom in numbers sufficient to color the surface of ponds and ditches green (E. viridis) or red (E. sanguinea).