The properties of two classes of behavioral mutants of Escherichia coli (called tsr and tar) are described. The mutations in these strains define two complementary pathways of information flow in bacterial chemotaxis: behavioral responses to one set of stimuli are defective in tsr mutants, while responses to a complementary set of stimuli are defective in tar mutants. A double mutant containing both genetic lesions is defective in responses to all stimuli tested. The behavioral defects are shown to correlate with alterations in the properties of a methylation reaction involved in chemotaxis. Two independent sets of methyl-accepting proteins are demonstrated in the wild type, each set functioning in one of the two pathways mentioned above. Methylation of one set of proteins is defective in tsr mutants, while methylation of the complementary set is defective in tar mutants. The double mutant shows no methylation of either set. The relationship between the genetic loci (tsr and tar) and the methyl-accepting proteins is discussed.