An indole-requiring (Ind − ) mutant of Salmonella typhimurium , isolated from a culture of a leaky trpA mutant, was genetically analyzed by P22-mediated transduction. The mutation site giving the Ind − phenotype was shown to be in trpB , the second gene of the trp operon. A second mutation at this site resulted in change of nutritional requirement from indole to anthranilic acid (Anth − ). This phenotype is normally associated with mutations in the first trp gene, trpA . However, the Anth − mutant also excreted anthranilic acid and showed “self-feeding” on unsupplemented media. Of two possible explanations for this aberrant phenotype, the first, that the trpB mutations may be in the “unusual” region, was dismissed on genetic evidence and on the biochemical evidence that an active anthranilate synthetase (AS) is produced. The alternative explanation, that the affected enzymatic activity, phosphoribosyl transferase, is unstable in vivo, but its AS component 2 activity is stable, is considered more probable.