Thiothrix is a genus of filamentous sulfur-oxidizing bacteria, related to the genera Beggiatoa and Thioploca. They are usually Gram-negative (but can be Gram-variable) and rod-shaped (0.7–1.5 µm in width by 1.2–2.5 µm in length). They form ensheathed multicellular filaments that are attached at the base, and form gonidia at their free end. The apical gonidia have gliding motility. Rosettes of the filaments are not always formed but are typical. Sulfur is deposited in invaginations within the cell membrane.