The induction of an IAA-destroying enzyme in Arthrobacter sp. that can utilize IAA as its sole source of carbon and nitrogen was investigated.1. The enzyme was most effectively induced by 10−³ to 2x10−³ M IAA, at pH 6.5.2. All tested IAA analogs were unable to induce the enzyme. Analogs other than indole-3-lactic acid were rather inhibitory on the induction with IAA.3. The induction period was shortened with the age of culture in both polypeptone and acetate media.4. Pretreatment of the bacterium with IAA caused a shortening of the induction period.5. The induction was inhibited by various antibiotics, amino acid analogs and nucleobase analogs.6. The induction was less remarkable in actively proliferating cells than it was in slowly proliferating ones.