Majusculamide C, a novel cyclic depsipeptide from the deep water variety of Lyngbya majuscula that inhibits the growth of a number of fungal plant pathogens, has been structurally characterized. It consists of seven α-amino acid units, one β-amino acid unit, and one hydroxy acid unit. The structures of the units were determined by acid hydrolysis to glycine, L-alanine, L-N-methylvaline, L-N,O-dimethyltyrosine, L-N-methylisoleucine, racemic 2-amino-4-methylpentanone, 3-amino-2-methylpentanoic acid of undefined stereochemistry, and N-[2(S)-hydroxy-3(S)-methylpentanoyl]glycine. Sequencing of the units implied from the hydrolysis product structures was achieved by mass spectral and proton NOE studies, with support from elemental analysis, 13C and 1H NMR, IR, and UV spectral data.