We previously showed that a mutant strain of Salmonella typhimurium completely deficient in both the chemoreceptor methylating (CheR) and demethylating (CheB) enzymes can still exhibit chemotaxis to aspartate and other attractants (J. Stock, A. Borczuk, F. Chiou, and J. E. B. Burchenal, Proc. Natl. Acad. Sci. USA 82:8364-8368, 1985). We used this cheR cheB mutant to examine the possibility of an additional requirement for S-adenosylmethionine in chemotaxis besides its role in chemoreceptor methylation. A metE mutation was transduced into a cheR cheB double mutant, and the cells were starved for methionine. Despite the fact that intracellular S-adenosylmethionine dropped from approximately 100 microM to less than 0.2 microM, chemotaxis was largely unaffected. In contrast, a corresponding cheR+ cheB+ metE mutant completely lost its chemotaxis ability after being starved for methionine. We conclude from this observation that the primary requirement for S-adenosylmethionine during bacterial chemotaxis is in the methylation of receptor proteins.