A series of novel extracellular siderophores, termed carboxymycobactins, have been isolated from M. awum as iron complexes and their structures determined. They are the first cxtraccllular siderophorcs to be charactcrised from pathogenic mycobacterm and are analogous to their intraccllular mycobactin counterparts, except that their Rl alkyl sidcchain is shorter and terminates in a carboxylic acid, resulting in much greater hydrophilicity and explaining their occurrence in the aqueous extraccllular medium.