The Nostocales order contains most of the species of cyanobacteria. It includes filamentous forms, both simple or branched, and both those occurring as single strands or multiple strands within a sheath. Some but not all members show a decrease in width from the base. Again, some but not all have heterocysts.

The families included are:

• Microchaetaceae
• Nostocaceae
  Nostocaceae
  The Nostocaceae is a family of cyanobacteria that forms filament-shaped colonies enclosed in mucus or a gelatinous sheath. Some genera in this family are found primarily in fresh water , while others are found primarily in salt water . Other genera The Nostocaceae is a family of cyanobacteria...
  
  
• Rivulariaceae
  Rivulariaceae
  Rivulariaceae is a family of cyanobacteria within Nostocales in which the filaments are tapered from wider at the base to narrower at the tip. The type species is Rivularia haematites C. A. Agard.-External links:* * * *...
  
  
• Scytonemataceae