The transformable strain of Bacillus subtilis strain 168 is extremely susceptible to growth inhibition by d-tyrosine. The molecular events associated with the inhibition of growth by d-tyrosine in this strain include the false feedback inhibition and probably the false repression of prephenate dehydrogenase. These effects were found to contribute to the formation of d-tyrosine-containing proteins by decreasing the intracellular concentration of l-tyrosine. Accordingly, growth inhibition of strain 168 by the d isomer of tyrosine was shown to be progressive, enduring, and delayed by prior growth on l-tyrosine. The synthesis of cellular macromolecules and viable cell count were progressively diminished in d-tyrosine-inhibited cultures. Several different enzyme activities were reduced after growth in the presence of d-tyrosine. Isotopic d-tyrosine was incorporated into cellular proteins without change of optical configuration. Long chains of cells with completed septa were observed microscopically, and therefore some cell wall effect may also be implicated.