Taxonomic revision of the traditional polyphyletic cyanobacterial genus Phormidium is based on molecular sequencing combined with the definition of distinct, autapomorphic features. Several genera, clearly separated from each other (with genetic similarity lower than 95%), were already defined and separated from this widely conceived generic unit (Phormidesmis, Wilmottia and others). All of these new generic taxa are characterized by morphological markers also. We have studied another group of species (mainly the strains from salterns in Thailand), which was classified earlier in the genus Phormidium, but it represents an isolated cluster according to 16S rRNA gene sequencing and is characterized by specific uniform and morphological features. Because this whole group represents a phylogenetically and morphologically distinctly separated cluster (see Phormidium group I sensu Komárek & Anagnostidis 2005), we describe it as a special taxon Oxynema genus novum, in agreement with the Botanical Nomenclatoric Code (ICBN; McNeill & al. ed., 2007). The genetically most related clusters always have genetic similarity less than 93% and differ by distinct autapomorphic features. The filaments of members of Oxynema are cylindrical, narrowed and bent at the ends, commonly attenuated to a terminal elongated, more or less sharply pointed cells without calyptra. Thylakoids in cells are distinctly radially arranged, similarly as in the genera Microcoleus and “Phormidium autumnale”-type. The ecology of all members, which belong potentially to these types, is also similar: all species from this cluster were recorded from halophilic habitats, less frequently from thermal springs and soil biotopes with higher salt contents.