Part I deals with the fine-structure of 25 marine and 3 freshwater strains of cryptomonads, and 2 species observed in fixed marine plankton samples. Emphasis is placed on scanning electron microscope observations, which yield valuable taxonomic data, providing that cell shrinkage (up to about 25 % linear), and other undesirable effects of preparation schedules, are taken into account. The fine-structural information is used for judging the taxonomic value of some cell features. Characters having taxonomic significance at various levels include the cell shape, periplast features (especially the shape and size of the periplast areas), chloroplast number, nucleomorph position, presence of a 'tail', and the presence of a true, non-artifactual furrow on the ventral cell face. A new classification system is proposed. This allows for the existence of three orders, four families, seven pigmented genera, and two colourless ones. Most diagnosis or descriptions of taxa above the generic level are based on only one character. This should make the system capable of accomodating new taxa with little difficulty. The following new taxa are described: Rbinomonas reticulata comb. nov., R. . reticulata var. atrorosea comb. et stat. nov., R. reticulata var. compresses var. nov., R. reticulate var. eleniana var. nov., Proteomonas pseudobaltica comb. nov., P. pseudobal tics var. 1 eonardiana var. nov., Pyrenomonas sahne var. curvata var. nov., Pyrenomonadales ord. nov., Pyrenomonadaceee fam. nov., and Cryptomonadales ord. nov. - Part II deals with the diatom genus Mastoglola. An update of the taxa assigned to this genus is given, together with a 'resemblance list' useful for identifying taxa not included in the main reference work for the genus. The frustule architecture of Mastoglola smitbii is examined with particular reference to the valvocopula, especially the features involved in its integration with the valve.