SummaryThe Euglenineae form a well‐defined natural group comprising a great number of species, the subdivision of which into families is difficult to make.Among taxonomic features, the basic shape of the cell body is fusiform but often irregular and twisted, with periodical tapering towards the posterior end. It may be concealed by morphological aberrations. The metaboly of many species and the occurrence of dimensional varieties render the use of shape and size as specific features sometimes rather equivocal. Other features have to supplement them for the identification of the species.The shape of the nucleus is sometimes characteristic, but the structure of the plastidome and the presence or absence of pyrenoids are of much greater taxonomic importance. These features vary greatly in the species ofEnglena, while the other green genera have almost uniform chromatophores. By plastid and other characteristicsTrachelomonasandColaciumare related to certain species ofEuglena, whilePhacusandLepocinclisare nearer to others.The flagellar apparatus of the Euglenineae is tentatively considered as being composed of two flagellar units throughout the group, the length alone varying according to the genus.Eutreptiawith two equal or more or less unequal flagella would be nearest to the hypothetical ancestral form. The other green genera would be derived from it by a further shortening of the minor flagellum. Its near convergence to the long active flagellum gives the impression of bifurcation.Distigmawould be the apoplastidic counterpartof Eutreptia, Astasiathat ofEuglena.All green Euglenineae and some colourless ones have an eye‐spot and opposite to it a thickening of the active flagellum or flagella not found in the species without an eye‐spot.The striation of the periplast varies greatly, without correlation to the degree of cell metaboly. A first attempt at classifying the different types of metaboly is made. Envelopes differ from the periplast by being exudations of inorganic substances.While the distinction between the phototrophic and the saprotrophic Euglenineae seems gradually to disappear, the holozoic forms are more distantly derived. There are no indications as to their evolution, as no zootrophic green Euglenineae are known, and some are similar to saprotrophic forms. The entire class of the Euglenineae is rather uniform, and a rational and at the same time natural classification has not yet been att