The ability of E. coli to adapt to constant levels of attractant and repellent chemicals was studied by examining the patterns of flagellar movement in cells subjected to abrupt concentration changes. Wild-type bacteria exhibited transient responses to such stimuli, in support of previous findings. Nonchemotactic mutants of the cheX class responded to both attractants and repellents, but were unable to terminate these behavioral changes as long as the stimulating chemical was present. The sensory adaptation defect of cheX strains may be due to an inability to methylate several cytoplasmic membrane proteins that initiate changes in flagellar movement in response to chemoreceptor signals. Based on these results, possible mechanisms of stimulus transduction and sensory adaptation during chemotaxis are discussed.