Secondary mutants able to utilize d-histidine, dhu, were isolated in histidine auxotrophs of Salmonella typhimurium. Mutations of one class (dhuA) are closely linked with the hisP locus which codes for a component of histidine permease. The specific activity of l-histidine permeation was estimated as increased two- to seven-fold in dhuA mutants. The dhuB mutants which have not been mapped also had elevated specific activity of l-histidine permeation. The uptake of d-histidine, barely detectable in the parental strains, was prominent in dhuA mutants and showed an apparent Michaelis constant about 1,000-fold higher than that observed with l-histidine. No change was detected in the kinetics of l-histidine permeation. d- and l-histidine competed in the uptake process. Tertiary mutants which lost the ability to grow on d-histidine were isolated by ampicillin counter-selection in dhuA his(-) strains. All of them mapped in the dhuA hisP region. Most of them had all known properties of hisP mutants. It is inferred from these data that the dhuA mutations increase synthesis of components critical to d- and l-histidine permeation.