Streptomyces scabies is the cause of the economically important disease potato scab, and a pathogen of tap and fibrous roots. A previous study indicated that Streptomyces can synthesize indole-3-acetic acid (IAA) through the Indole-3acetamide (IAM) pathway (Manulis et al., 1994). Two enzymes, tryptophan monoxygenase (IaaM) and indole-3-acetamide hydrolase (IaaH), participate in IAA synthesis via the IAM pathway. Based on homology to other IAM and IAH amino acid sequences we identified candidate iaaM (SCAB75511) and iaaH (SCAB75501) genes in the completely sequenced genome of S. scabies 87-22. The function of the candidate genes in the IAA biosynthetic pathway was evaluated by creating the deletion mutants iaaH, iaaM5 and iaaM8. IAA production by all three mutant strains was lower than production by the wild type strain (87-22). When inoculated onto radish seedlings, all three mutant strains were reduced in virulence relative to the wild type strain, as measured by root necrosis. Genetic complementation of iaaM5 and iaaM8 deletion strains partially restored both IAA production and virulence on radish seedlings. These results suggest that the iaaM (SCAB75511) and iaaH (SCAB75501) genes are IAA biosynthetic genes and that they contribute to virulence of S. scabies through the synthesis of IAA as a virulence factor.