Medium length, non-motile filaments (20-200 µm). Straight or smoothly curved filaments with no branching. Cells are rectangular (0.7 x 1.2 µm) and a clear cell septa is present without indentations at the septa. Filaments are found radiating outwards from the floc structure causing inter-floc bridging. The filament staining is Gram negative or Gram variable when sulfur granules are present and Neisser negative with Neisser positive granules observed frequently. Exhibits bright sulfur granules in the presence of sulfides under phase contrast (use the S-test). Poly-ß-hydroxybutric acid (PHB) is frequently observed as dark intracellular granules. Small amounts of attached growth may occur when not growing. Can form rosettes and the filaments can have gonidia on the tips. Rosettes are when many filaments radiate outward from a common origin. Difficult to detect its thin sheath. Easy to identify due to its highly refractive sulfur granules.

Type 021N is similar when in the bulk solution and with no attached growth, although Type 021N has no sheath.

This filament is usually found in environments where there are limited nutrients (Nor P). It can also be found in wastes containing specific compounds with sulfides and/or organic acids or environments with low DO. Sometimes found in plants with high pH in the aeration system..

RAS chlorination should be definitely used if in the rapid growth phase. After chlorination, sheath may still be present and interfere with SVI. Increase DO levels, add nutrients or lowering the pH according to the problem. Septic wastes can be remediated by pre-aeration or pre-chlorination. Nutrient deficient wastes can be checked by effluent values of residual NH³ and o-PO⁴ and should be supplemented if necessary.

Thiothrix ranks 5th in number of predominance.