The observations to be recorded here have been carried out primarily for the purpose of clarifying the taxonomic position and naming of the small organism previously known as Chromulina pusilla Butcher. This had been shown (Manton, 1959a) to possess the pigments and fine structure appropriate to a position in or near the Chlorophyceae but quite inappropriate to a position within the Chrysophyceae to which the genus Chromulina properly belongs. Before selecting a new generic name, it was highly desirable to make some comparative electron microscopical observations on authentically named material of appropriate kinds since the fallibility of the light microscope for study of organisms of this order of size (1–3µ) had been all too clearly exposed. A search was therefore made for a representative or representatives of the only genus of green flagellates known to us from the literature to possess a single posteriorly directed flagellum, namely Pedinomonas.