Cyanobacterial biodiversity is paradoxical, since they strongly vary in cytological characters being metabolically uniform. Cyanobacterial phylogeny is also paradoxical, since structural genes of rRNA are too conservative for a large phylum. On the paradoxical evolutionary tree, neighbors with strongly contrast phenotypes reside, while objects with alike phenotypes demonstrate heterological structure of 16S rDNA. Cyanobacterial systematics is paradoxical too, since it is logically contradictory (on the one hand, phylum BX Cyanobacteria generally separates itself on molecular-biological grounds; on the other hand, in accordance with traditional botany algorithm which is used in the classification of blue-green algae, this phylum artificially subdivides itself in morphological groups (ultrastructural characters are taken into account in rare instances only). The unique peculiarity of cyanobacterial taxonomy (with rare exceptions, e.g. Cyanobacterium stanieri) is its general non-usage of the category of species;