[reaction: see text] Bioassay-guided investigation of the extract from a Lyngbya majuscula/Schizothrix sp. assemblage of marine cyanobacteria led to the discovery of somocystinamide A (1), an extraordinary disulfide dimer of mixed PKS/NRPS biosynthetic origin. Somocystinamide A (1) was highly acid-sensitive, rapidly and completely converting to a characterizable derivative (2). Compound 1 exhibits significant cytotoxicity against mouse neuro-2a neuroblastoma cells (IC50 = 1.4 microg/mL), whereas 2 has no activity.