Unicellular cyanobacteria constitute a substantial, ecologically important part of freshwater and marine microflora. Solitary, elongated cyanobacterial cells without apparent slime envelopes and dividing in a single plane perpendicular to the longitudinal axis were traditionally classified into the genus Synechococcus. The type species originates from a freshwater benthic habitat while a number of other Synechococcus-like species were described from diverse environments. Morphologically similar, unicellular populations of "Synechococcus" belong to the most abundant oxygen evolving prokaryotes inhabiting freshwater and oceanic picoplanktic communities. Other species from extreme thermal habitats were described from temperatures over 70 degrees C. Recent molecular analyses, particularly the 16S rRNA gene and other multi-locus gene based phylogenies of Synechococcus-like species reveal the polyphyly and phylogenetic diversity of such simple cyanobacteria emphasizing the necessity of their accurate polyphasic taxonomic re-classification. To better understand the phylogeny of different Synechococcus-like species, we constructed a genome-derived phylogenetic tree using 133 cyanobacterial and eubacterial genomes as well as a 16S rRNA gene phylogenetic tree. Both trees indicate existence of deep splits between individual Synechococcus lineages corresponding with indistinct ecological and cytomorphological differences. The generic units derived from the traditional concept of the genus Synechococcus based on the combined traits and markers must be defined and validly described. The comparative taxonomic classification of phylogenetic clusters corresponding to various morphotypes and genotypes similar to the genus Synechococcus (Cyanobium, Parasynechococcus, Picosynechococcus, Prochlorococcus, Thermosynechococcus, Thermostichus) are discussed in the article.