Herbaspirillum seropedicae is a plant-growth-promoting bacterium associated with several plants, including economically important graminaceous species, such as rice, sugarcane, wheat sorghum and maize. The mechanisms involved in the interaction between H. seropedicae and these plants are not completely understood. It has been proposed that a main factor responsible for bacteria enhancing plant growth is the production and secretion of phytohormones such as the auxin indole-3-acetic acid (IAA) by microorganisms. H. seropedicae is able to produce IAA which could affect plant morphology and development. Analysis of the genome sequence of H. seropedicae (The GENOPAR PROGRAM) revealed at least three possible pathways of IAA synthesis. To understand how IAA production affects plant physiology, we produced a mutant of the gene iaaH, which codes for indole-3-acetamide hydrolase, a key enzyme of IAA biosynthesis. The plasmid pRAMiaa1 containing the iaaH gene was disrupted by inserting the transposon EZ:TN <TET-1> (containing a gene for tetracycline resistance). H. seropedicae strain SmR1 was then transformed with pRAMiaa1 and the mutant strain RAMiaa1 was produced by homologous recombination. However, this mutant strain RAMiaa1 produced IAA when grown in the presence of tryptophan, indicating that indole-3-acetamide hydrolase was not essential for IAA production. We conclude that probably other IAA biosynthetic pathways are compensating for the loss of the indole-3-acetamide hydrolase.