Fischerellin A, the most active allelochemical compound of Fischerella muscicola that is directed against other cyanobaceria and photoautotrophic organisms, is a potent photosystem-II inhibitor of novel structure. Structural elements are an enediyne moiety and two heterocyclic ring systems. Fischerellin A is unique and unrelated to all known cyanobacterial metabolites, it exhibits a MIC of 14 nM against Synechococcus ~x: 6911. and has interesting antifungal and herbicidal activity.