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We report here on a new class of siderophores isolated from Rhodococcus erythropolis IGTS8, the first structurally characterized from any species of Rhodococcus and for which we suggest the name heterobactins. These siderophores consist of a tripeptide of sequence (N-OH)-L-Orn-Gly-D-Orn-(δ-N-dihydroyxbenzoate). The alpha amino group of the D-Orn is derivatized either as a 2-hydroxybenzoxazolate in heterobactin A or remains free in heterobactin B. The structures were determined by a combination of amino acid analysis, mass spectrometry and NMR methods. The two new compounds are true siderophores in that they relieve iron limited growth in the producing strain. The heterobactins are also transported by other non-producing bacteria. Growth promotion tests using various transport mutants revealed that in E. coli heterobactin A is only recognized by the catecholate receptor Cir while heterobactin B is taken up in both E. coli and A. flavescens JG9 via a hydroxamate transport system.