Relatively long , non-motile filaments (100-1000 µm). Straight or smoothly curved with tree-like false branching. The cells are round-ended and rod shaped (1.0-1.8 x 1.5-3.0) and are contained in a clear, tightly fitting sheath. Note: They can be rectangular when the cells are tightly packed within the sheath. The cell septa are clear and easily observable with indentations. Filaments radiate outward from the floc surface into the bulk solution and can cause sludge settling interference by inter-floc bridging. The filament is usually Gram negative and Neisser negative. There are no sulfur granules. Poly-ß-hydroxybutric acid (PHB) is frequently observed as dark intracellular granules. In wastewater that is nutrient deficient, an exocellular slime coat may be present. Attached growth is usually uncommon, but may occur when at low growth rate.

Type 1701 may be related, but the diameter is much smaller.

This filament is usually found in environments where there is low DO or low nutrients (Nor P)

RAS chlorination can be used to get rid of the filaments but process changes should also be made. Cell lysis occurs readily on this type of filament, although the empty sheaths still remain. Sludge wasting is necessary to remove them entirely from the system. Manipulation of F/M and DO concentration can be used to control the filaments. Nutrient deficient wastes can be checked by effluent values of residual NH³ and o-PO⁴ and should be supplemented if necessary.

Sphaerotilus natans ranks 6th in number of predominance. Typically not found in pulp-mills with activated sludge.