Legionella dumoffii is one of the causative agents of Legionnaires' disease. There are 50 species in the genus Legionella, of which 10 species including L. dumoffii are known to exhibit uncharacterized blue-white autofluorescence. We constructed an L. dumoffii strain that exhibited a high intensity blue-white fluorescence, isolated a fluorescent pigment from the strain, and determined its molecular formula to be C19H14O3 by high-resolution mass spectrometry. An NMR analysis revealed that this was a new isocoumarin compound, which was named legioliulin. (C) 2004 Elsevier Inc. All rights reserved.