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. The absolute configuration of the novel amino acid 2-methyl-3-aminopentanoic acid (MAP), a component of the structure of 1, has been determined to be 2S,3R. Complex cyclic peptides and depsipeptides have emerged as an important new class of metabolites present in extracts of marine sponges (1-13). Many of these sponge peptides have been found to be extremely potent cytotoxins (5-9) and/or enzyme inhibitors (10-13). The apparent structural relationships between some of the sponge peptides and peptides isolated from blue-green algae and other microbial sources has prompted several authors to raise the possibility that peptides which they have isolated from sponges are actually microbial products (2,4,10,12). As part of an ongoing study of biologically active metabolites from tropical marine invertebrates (14,15), we have isolated the cyclic depsipeptide majusculamide C [1] from the sponge Ptilocaulis trachys DeLaubenfels (class Demospongiae, order Axinellida, family Axinellidae) collected at Enewetak Atoll in the Marshall Islands. Majusculamide C [1] (16) and 57-normajusculamide C [2] (17) were first reported from Lyngbya majuscula, a toxic blue-green alga also collected at Enewetak Atoll. Both 1 and 2 have been shown to exhibit antifungal activity against pathogens of commercially important plants. Dolastatins 11 [3] and 12 [4], two closely related cyclic depsipeptides isolated from the Indo-Pacific sea hare Dolabella auricularia, were found to exhibit promising cytotoxic activity (murine leukemia P388 ED50, 2.7 × 10^-8 and 7.5 × 10^-8 kg/ml, respectively) (18). The biological activities of the majusculamides and the dolastatins make them attractive synthetic targets. In order to facilitate the synthesis of these peptides, we have determined the absolute configuration of the novel amino acid 2-methyl-3-aminopentanoic acid (MAP) in majusculamide C [1]. The configuration of the MAP residue was not elucidated in the original investigations of the majusculamides (16,17) or the dolastatins (18).