Genus of single-celled organisms in the protozoan phylum Sarcomastigophora that live in fresh water. They are usually oval or cigar-shaped, less than 0.5 mm/0.2 in long, and have a nucleus, green pigment in chloroplasts, a contractile vacuole, a light-sensitive eyespot, and one or two flagella, with which they swim. A few species are colourless or red.

Classification
Euglena are members of the order Euglenida in class Phytomastigophora, subphylum Mastigophora, phylum Sarcomastigophora.

This organism combines animal and plant characteristics. Its plantlike characteristics include the chloroplasts and its consequent ability to photosynthesize, and the rigid cellulose wall of some species. Its main animal characteristic is its motility, but it can also absorb food from its environment, and many species have a flexible body covering.