Siderophores are relatively low-molecular-weight compounds that typically have a very high affinity constant (1025-1050) for iron (Fe3+), which is an essential element for most microorganisms owing to its importance in a variety of biochemical reactions. The role of siderophores is to scavenge extracellular iron from the environment and transport it into microbial cells. Hundreds of siderophores have been isolated from terrestrial, especially pathogenic, microorganisms, and their biosyntheses and iron-sequestering systems have been enthusiastically studied. In spite of the high abundance of iron in the earth's crust, the dissolved iron concentration is particularly low (20pM-1nM) in the surface water of the open ocean. In such an iron-deficient environment, marine bacteria are thought to get iron by siderophore-based iron-sequestering systems. However, only a few studies concerning siderophores of marine bacteria have been done. In the course of screening for new siderophores from marine bacteria, we found that Pseudoalteromonas sp. KP20-4 produced new siderophores. In this note, the isolation and structural determination of pseudoalterobactin A (1) and B (2), excreted by marine bacterium Pseudoalteromonas sp. KP20-4, are reported. The molecular formula of pseudoalterobactin A (1) was C41H63N11O21S, established from 13C-NMR and HR-FAB-MS data, and that of pseudoalterobactin B (2) was C41H63N13O21S. Structural determination using 2D NMR spectra including COSY, TOCSY and HMBC, and ESI-MS/MS data revealed that the only structural difference between 1 and 2 was the replacement of Lys(2) in 1 by Arg in 2. The CAS assay showed that the pseudoalterobactins exhibited strong activity comparable to that of enterobactin. Both 1 and 2 exhibited an ED50 value, the concentration that reduced the absorbance at 630nm of the CAS solution by 50% in 2 hours, of 20μM under our assay conditions. Enterobactin and desferrioxamine B exhibited ED50 values of 60μM and 500μM, respectively, under the same conditions. REID et al. have reported the marine siderophores, alterobactins A and B, which had exceptionally high affinity for the ferric ion (affinity constant of 1049-1053). They speculated that this high affinity was derived from the coordination by a catechol and two β-hydroxy-Asp residues with the ferric ion. The structural resemblance between these alterobactins and pseudoalterobactins 1 and 2, having a catechol and two β-hydroxy-Asp residues, led us to presume that the pseudoalterobactins would also have extraordinary affinity to the ferric ion.