An ampicillin enrichment technique was used to isolate 39 nicotinic acid-requiring mutants of Salmonella typhimurium LT-2. Using interrupted-mating and transductional mapping procedures, three loci, designated nadA, nadB, and nadC, were identified. These loci mapped at 33, 82, and 6 min, respectively, on the S. typhimurium linkage map. The arrangement of the loci on the Salmonella linkage map corresponded closely to the nadA, nadB, and nadC loci on the Escherichia coli K-12 linkage map, indicating that the de novo pathway to nicotinamide adenine dinucleotide and the genes governing the enzymes involved in this pathway in S. typhimurium are very similar to those in E. coli. Evidence is also presented which indicates that the product of the nadC locus in S. typhimurium LT-2 is the enzyme quinolinic acid phosphoribosyltransferase. All nadC mutants of S. typhimurium secreted between 2 and 8 mumol of quinolinic acid per 100 ml of secretion medium. In addition, none of the nadC mutants isolated were able to grow in 10(-3) M quinolinic acid, whereas all nadA and nadB mutants of S. typhimurium grew well in the presence of quinolinic acid. Transductional crosses between nadB mutants provided evidence suggestive of more than one locus in the nadB region.