The large crescents, with their unique morphology, still present many puzzles in their systematics. It is already clear from ultrastructural features that the genus "Selenomonas" is most probably an artificial classification, bringing together possibly unrelated organisms, simply because of their common possession of crescent morphology and peculiar flagellar insertion location. Successful attempts to maintain the large crescents in continuous culture over short terms have been reported by Prins but long term culturing has not been possible so far. Genetic sequencing of the large crescents should provide the essential information required to better understand and classify these organisms.