Chemical examination of two collections of the Caribbean cyanobacterium, Lyngbya majuscula, led us to isolate several antimicrobial agents. The most active was elemental sulphur, followed by the previously reported fatty acid (-)-7(S)-methoxytetradec-4(E)-enoate, and then by two new amides of this fatty acid, malyngamide D and malyngamide D acetate. The new compounds possess vinyl chloride and α,β-unsaturated enone functional groups. The structures of the new compounds are based on spectroscopic evidence including several 2D NMR techniques at high field as well as chemical interconversion.