A genetic locus, aroT, located between chr and the trp operon in Salmonella typhimurium, and similar genes, aroR and aroS, near the trp locus of Escherichia coli, were found to be involved in the transport of aromatic amino acids. Genetic lesions at these loci cause a variable diminution in uptake and accumulation of aromatic amino acids, alanine and glycine compared with the wild type. The F'trp episome carries the aro R locus. Curing an E. coli strain of the F'trp episome which covers a chromosomal deletion from cysB through the trp operon and tonB regions, results in a 60 to 80% decrease in tryptophan uptake. The introduction of F'trp into a trp operon-deleted S. typhimurium of low transport ability restores transportability, suggesting that aroT in this organism may be homologous with aroR in E. coli. In E. coli, tryptophan accumulation is normally increased by prior growth in L-tryptophan, while in S. typhimurium it is repressed. In both genera, the trpR gene appears to have no effect on the tryptophan transport capabilities in response to changes in the concentration of L-tryptophan in the medium. Tryptophan transport in the S. typhimurium F'trp hybrid was subject to repression, while in the E. coli strain which carries F'trp covering the equivalent chromosomal delection, an increase in tryptophan accumulation was shown after growth in L-tryptophan supplemented medium.