Abstract Taxonomic diversity of desmids and other coccoid green algae is discussed in
relation to diVerent species concepts. For want of unambiguous criteria about species
delimitation, no reliable estimations of global species richness can be given. Application of
the biological species concept is seriously hampered by lack of sexual reproduction in
many species. Molecular analyses demonstrated cases of close aYliation between morphologically
highly diVerent taxa and, contrary, examples of little relationship between morphologically
similar taxa. Despite the fact that desmids and chlorococcal algae, because of
their microbial nature, can be readily distributed, cosmopolitan species are relatively
scarce. The geographic distribution of some well-recognizable morphospecies is discussed
in detail. Of some species a recent extension of their area could be established, e.g., in the
desmids Micrasterias americana and Euastrum germanicum, and in the chlorococcaleans
Desmodesmus perforatus and Pediastrum simplex.