Azospirillum brasilense responds to a spatial gradient of oxygen by formation of a band in the zone of optimal oxygen concentration. The same behaviour was observed in a ‘redox’ gradient of tetramethyl-p-phenylenediamine (TMPD) oxidized by ferricyanide, under conditions where oxygen uptake was inhibited by cyanide. Inhibitors of the bc1-complex of the redox chain did not inhibit aerotaxis. Cells inhibited by a high cyanide concentration still showed slow motility and weak attraction to oxygen, possibly as a result of cyanide-resistant respiration (3–5% of the initial level). It is suggested that electron flow through the redox chain (and/or consequent changes in protonmotive force), rather than binding of oxygen to terminal oxidases, may be a signal in the aerotactic response of A. brasilense.