The addition of α-methylmethionine to exponentially growing cultures ofSalmonella typhimuriumorEscherichia coliresulted in an immediate cessation of growth with low concentrations of the analogue. After a lag period, the length of which was dependent on the optical density of the culture, growth resumed at a rate characteristically slower than that of an untreated culture. Chemical analyses showed that the rate of synthesis of ribonucleic acid and protein byS.typhimuriumhad a pattern similar to the curve for optical density. However, deoxyribonucleic acid and cell numbers continued to increase at the normal rate for about 30 minutes, then slowed, and changed to the rate characteristic of cells treated with the analogue. Incorporation of thymine-14C, uracil-14C, and leucine-14C by the thymine auxotrophE.coli15T−confirmed the chemical analyses. After 25 minutes the cells of the culture treated with α-methylmethionine had incorporated only 15% of the uracil and 26% of the leucine as that of a control culture. The optical density was 30% that of a control culture. Thymine incorporation was 92% that of a control culture.