Callipeltin A (1) is a cyclic depsidecapeptide from a shallow water sponge of the genus Callipelta (order Lithistida), collected in the waters off New Caledonia. The structure of callipeltin A (1), which possesses the N-terminus blocked with a beta-hydroxy acid, and the C-terminus lactonized with a threonine residue, was determined by interpretation of spectral data, chemical degradation, and evaluation of the amino acids obtained by acid hydrolysis. Along with four common L-, one D-, and two N-methyl amino acids, it contains three new amino acid residues : beta-methoxytyrosine (beta OMeTyr), (2R,3R,4S)-4-amino-7-guanidino-2,3-dihydroxyheptanoic acid (AGDHE), and (3S,4R)- 3,4-dimethyl-L-glutamine. Callipeltin A (1) has been found to protect cells infected by human immunodeficiency (HIV) virus.