The hexadentats 3,4-catecholate/citrate siderophore petrobactin is produced by the mammalian pathogens Bacillus anthracis and Bacillus cereus apparently to evade the immune protein siderocalin and mediate their iron acquisition during infection; it is essential for the pathogenicity of these organisms. Solution thermodynamic and kinetic measurements show that incorporation of 3,4-catecholate subunits into the siderophore structure does not enhance its affinity for iron as compared to hydroxamate units, but it promotes fast iron removal from human diferric transferrin. The function of the photoreactive ferric citrate moiety remains unexplored in pathogenic Bacilli; however, the soil organism Bacillus subtilis can utilize both ferric petrobactin and its photoproduct as iron sources and therefore carries a specific uptake system for this exogenous siderophore.