The structure of Pyoverdine Pa, the siderophore of Pseudomonas aeruginosa, was elucidated by degradation, by FAB mass spectrometry and by NMR; it consists of a linear octapeptide bound to a 2,3-diamino-6,7-dihydroxyquinoline derived chromophore. Pseudomonads are widespread bacteria, which are divided into five groups of genetic homology according to their rRNA-DNA hybridization (1). The fluorescent pseudomonads, which belong to group 1, release, in iron-deficient conditions, yellow-green water-soluble fluorescent pigments, strongly chelating Fe(III) ions, which are the siderophores of these bacteria (2-4). Among the fluorescent pseudomonads, Pseudomonas aeruginosa is of particular importance since it is pathogenic for weakened organisms (deeply burnt people, for instance). In iron-deficient conditions, P.aeruginosa (ATCC 15692, PA0 1) produces large amounts (ea. 40 mg/l) of a pigment, which we call "Pyoverdine Pa", and which was shown to be the siderophore of this bacterium (5). This substance was isolated according to published procedures (2) and further purified by HPLC on reverse-phase octadecylsilane columns as its Al(III) or Fe(III) complex. Its spectral data as well as those of its metal complexes are very similar to those reported for pseudobactin, the siderophore of Pseudomonas B-ZU (4).