A new 2-pyrrolidinone with a polyunsaturated side chain has been identified from the cyanobacterium Fischerella muscicola (Thuret). Its structure was elucidated by UV, NMR and mass spectroscopy. Derivatisation of the natural product and stereocontrolled synthesis of the derivative allowed the determination of the absolute configuration by means of chiral gas chromatography. The new compound, fischerellin B, shows algicidal properties.