RESUMEN
Almost since the creation of the genus Euglena (Ehrenberg), the taxa assigned to it have been separated, split apart, and reorganized into new genera based on morphological relationships, resulting in the creation of the genera Phacus (Dujardin), Lepocinclis (Perty), Astasia (Pringsheim), and Khawkinea ( Jahn and McKibben) based on intuitive methods. In an effort to assess the validity of these genera, we have used small subunit (SSU) rDNA data to generate a phylogenetic framework for these genera, with particular attention to the genus Euglena. Using the conserved sequence areas, we performed a phylogenetic analysis using parsimony, maximum likelihood, and distance methods. These different criteria have resulted in trees of the same topology. The euglenoid clade was composed of phagotrophic euglenoids at the base, which gave rise to phototrophs that in turn gave rise to osmotrophs. Among the photosynthetic taxa, the biflagellate form diverged prior to the uniflagellate form. Additionally, the need for a revision in the taxonomy of some of these genera was demonstrated. Currently, taxa from the photosynthetic genera Euglena, Phacus, and Lepocinclis do not form monophyletic clades, but are intermixed with each other as well as with the osmotrophic taxa, Astasia and Khawkinea.