Phormidium pseudpriestleyi (Anagnostidis et Komárek 1988, synonym: Oscillatoria priestleyi W et G S West 1911) is a species of cyanobacteria that belongs to the order Oscillatoriales.
It is characterised by forming filaments without akinetes or heterocytes.
Phormidium pseudpriestleyi is 3 µm wide barrel-shaped, shorter than wide and sometimes even round - over a thousand cells would fit on a pinhead.
The filaments - hair-like structures - are made from many of these barrel-shaped to round cells. The last cell at each end of a filament is always round, whereas within the filament the cells are mainly barrel-shaped. Filaments can be several millimetres long.
The filaments have no, or very slight, constrictions between cells and can be surrounded by a sheath.