A spontaneous ethionine-resistant mutant of Escherichia coli was found to be nonchemotactic. The mutant had low S-adenosylmethionine (SAM) synthetase activity and a greatly reduced SAM pool. Transductants regained both chemotaxis and ethionine sensitivity. A partial revertant was chemotactic and its SAM pool was substantially higher than in the mutant, though the revertant still had low SAM synthetase activity and was only slightly ethionine sensitive. The results support the hypothesis that SAM is required for a normal tactic response to a chemical stimulus.