Parallel chemical and phylogenetic investigation of a marine cyanobacterium from Panama led to the isolation of two new PKS-NRPS-derived compounds, viridamides A and B. Their structures were determined by NMR and mass spectroscopic methods, and the absolute configurations assigned by Marfey's method and chiral HPLC analysis. In addition to six standard, N-methylated amino and hydroxy acids, these metabolites contained the structurally novel 5-methoxydec-9-ynoic acid moiety and an unusual proline methyl ester terminus. Morphologically, this cyanobacterium was identified as Oscillatoria nigro-viridis, and its 16S rDNA sequence is reported here for the first time. Phylogenetic analysis of these sequence data has identified O. nigro-viridis strain OSC3L to be closely related to two other marine cyanobacterial genera, Trichodesmium and Blennothrix. Viridamide A showed antitrypanosomal activity with an IC50 of 1.1 microM and antileishmanial activity with an IC50 of 1.5 microM.