A new stereoisomer of malyngamide C, 8-epi-malyngamide C (1), and the known compound lyngbic acid [(4E,7S)-7-methoxytetradec-4-enoic acid] were isolated from a sample of Lyngbya majuscula collected near Bush Key, Dry Tortugas, Florida. The structure of 1 was determined by NMR and MS experiments. The absolute configuration of 1 was determined by selective Mitsunobu inversion of C-8 to give malyngamide C, as determined by NMR, MS, and comparison of specific rotation. Both 1 and malyngamide C were found to be cytotoxic to HT29 colon cancer cells (IC(50) 15.4 and 5.2 microM, respectively) and to inhibit bacterial quorum sensing in a reporter gene assay.