A freshwater cyanobacterium, Nostoc spongiaeforme TISTR 8169, produced and excreted a novel violet pigment, nostocine A (I), which had broad-spectrum of growth inhibitory activity. The chemical structure of 1 was determined as 6-methyl-1,3,6,7,8-pentaazabicyclo[4.3.0]nonan-l,3,5(7)-trien-9-one on the bases of physicochemical evidence and an X-ray crystallographic analysis.Cyanobacteria are well known to produce extracellular metabolites with a wide variety of biological activities including toxins, antibiotics, fungicides, and antineoplastic However, there are very few studies on the cyanobacterial extracellular pigments6 We have isolated a novel violet pigment which was produced and excreted into a medium by Nostoc spongiaeforme TISTR 8169, a filamentous N2-fixing cyanobacterium. The pigment named nostocine A (I) has an unique nitrogen-rich ring -structure and shows a growth inhibitory activity with broad-spectrum against various organisms and cultured cells. In this study, the chemical structure of 1 was elucidated on the bases of physicochemical properties and an X-ray crystallographic analysis.