A simple method is discussed for the isolation of pseudobactin B, a new siderophore produced together with pseudobactin by plant growth promoting bacterium Pseudomonas PD 30. Pseudobactin B differs from the known pseudobactin only by the nature of the dicarboxylic acid attached to the chromophore, which is succinic acid instead of succinamide. The structure of pseudobactin B was determined by a comparison of its ¹H NMR, UV, and mass spectra with those of pseudobactin. An HPLC assay for pseudobactin B and pseudobactin, quantification in fermentation beer, and a new efficient method for isolating them are proposed.