Studies of bacterial chemotaxis to oxygen (aerotaxis) over a broad range of oxygen concentrations showed that at high concentrations, oxygen was a repellent of Salmonella typhimurium, Escherichia coli, and some bacilli, whereas it is known that at lower concentrations (less than or equal to 0.25 mM dissolved oxygen), oxygen is an attractant. In a temporal assay of aerotaxis, S. typhimurium in medium equilibrated with air (0.25 mM dissolved oxygen) and then exposed to pure oxygen (1.2 mM) tumbled continuously for approximately 20 s. The oxygen concentration that elicited a half-maximal negative (repellent) response was 1.0 mM for both S. typhimurium and E. coli. The receptor for the negative chemoresponse to high concentrations of oxygen is apparently different from the receptor for the positive chemoresponse to low concentrations of oxygen, since the oxygen concentration that elicits a half-maximal positive (attractant) response in S. typhimurium and E. coli is reported to be 0.7 microM. Adaptation to high concentrations of oxygen, like adaptation to low concentrations of oxygen, was independent of methylation of a transducer protein. Only the response to low oxygen concentrations, however, was altered by interaction with the amidated Tsr transducer in cheB mutants.