An iron-binding compound was isolated from ethyl acetate extract of culture supernatant fluid of Rhizobium trifolii AR6 and was purified by iron-exchange chromatography. The compound was characterized by UV and IR. It contained 2,3-dihydroxy-benzoic acid and threonine and was accumulated during stationary phase of growth in iron-deficient media. Synthesis of the siderophore was repressed by FeCl3. In iron limited medium the compound promoted growth of R. trifolii strains.