Majusculamide C [1], a cyclic depsipeptide metabolite of the blue-green alga Lyngbya majuscula, has been isolated from the sponge Ptilocaulis trachys collected at Enewetak Atoll in the Marshall Islands. As part of an ongoing study of biologically active metabolites from tropical marine invertebrates, we isolated majusculamide C from the sponge. Majusculamide C and 57-normajusculamide C were first reported from Lyngbya majuscula and exhibit antifungal activity against pathogens of commercially important plants. Dolastatins 11 [3] and 12 [4], two closely related cyclic depsipeptides from the Indo-Pacific sea hare Dolabella auricularia, show promising cytotoxic activity (murine leukemia P388 ED50 2.7 × 10^-8 and 7.5 × 10^-8 kg/ml, respectively), making the majusculamides and dolastatins attractive synthetic targets. To facilitate the synthesis of these peptides, we determined the absolute configuration of the novel amino acid 2-methyl-3-aminopentanoic acid (MAP) in majusculamide C [1], which was not elucidated in the original investigations of the majusculamides or dolastatins. The absolute configuration of the MAP residue was determined to be 2S,3R.