A new type of bacterium "Candidatus Ovobacter propellens" is described. It is a large ovoid species with a number of unusual features including special intracellular membrane systems and a huge flagellar tuft consisting of about 400 flagella. "Ovobacter" lives in oxygen gradients in the surface layer of sulphidic marine sediments at an O(2)-tension of about 0.5% atmospheric saturation. It swims continuously and very fast (up to 1 mm s(-1)). Its mode of swimming and its chemosensory behaviour towards O(2) are described in detail.