Pseuhmonusfiorescens strain CHAO is an effective biocontrol agent against soil-borne fungal plant pathogens. In this study, indole-3-acetic acid (IAA) biosynthesis in strain CHAO was investigated. Two key enzyme activities were found to be involved: tryptophan side chain oxidase (TSO) and tryptophan transaminase. TSO was induced in the stationary growth phase. By fractionation of a cell extract of strain CHAO on DEAE-Sepharose, two distinct peaks of constitutive tryptophan transaminase activity were detected. A pathway leading from tryptophan to IAA via indole-3-acetamide, which occurs in Pseuhmonas syringtze subsp. savustanoi, was not present in strain CHAO. IAA synthesis accounted for 6 1.5% of exogenous tryptophan consumed by resting cells of strain CHAO, indicating that the bulk of tryptophan was catabolized via yet another pathway involving anthranilic acid as an intermediate. Strain CHA750, a mutant lacking TSO activity, was obtained after Tn5 mutagenesis of strain CHAO. In liquid cultures (pH 6.8) supplemented with 10 mM-L-tryptophan, growing cells of strains CHAO and CHA750 synthesized the same amount of IAA, presumably using the tryptophan transaminase pathway. In contrast, resting cells of strain CHA750 produced five times less IAA in a buffer (pH6-0) containing 1 mM-L-tryptophan than did resting cells of the wild-type, illustrating the major contribution of TSO to IAA synthesis under these conditions. In artificial soils at pH -7 or pH -6, both strains had similar abilities to suppress take-all disease of wheat or black root.rot of tobacco. This suggests that TSO-dependent IAA synthesis is not essential for disease suppression.