The novel cyclic depsipeptides papuamides A (1), B (2), C (3), and D (4) have been isolated from Papua New Guinea collections of the sponges Theonella mirabilis and Theonella swinhoei. Their structures were determined by a combination of spectroscopic analysis and chemical degradation and derivatization studies. In addition to glycine, alanine, and threonine, these peptides contain a number of unusual amino acids including 3,4-dimethylglutamine, β-methoxytyrosine, 3-methoxyalanine, and 2,3-diaminobutanoic acid or 2-amino-2butenoic acid residues. Papuamides A-D (1-4) are also the first marine-derived peptides reported to contain 3-hydroxyleucine and homoproline residues. These peptides also contain a previously undescribed 2,3-dihydroxy-2,6,8-trimethyldeca-(4Z,6E)-dienoic acid moiety N-linked to a terminal glycine residue. Papuamides A (1) and B (2) inhibited the infection of human T-lymphoblastoid cells by HIV-1 RF in vitro with an EC 50 of approximately 4 ng/mL. Compound 1 was also cytotoxic against a panel of human cancer cell lines with a mean IC 50 of 75 ng/mL. Sponges in the genus Theonella (order Lithistida) have been a prolific source of structurally diverse, biologically active peptides. 1 Recent examples of Theonella peptides include polytheonamides, 2 cyclotheonamides, 3 theonellapeptolides, 4 the-onellamides, 5 theonegramides, 6 keramamides, 7 mozamides, 8 mutoporins, 9 microsclerodermins, 10 cupolamide, 11 oriamide, 12 and cyclolithistide A. 13 The biosynthetic origin of compounds isolated from Theonella sponges has often been attributed to symbiotic bacteria or microalgae. Recently, the peptide metabolite theopalauamide 14 was found localized in eubacteria that live symbiotically within Theonella swinhoei. Many Theonelladerived peptides demonstrate potent cytotoxicity, 2,5,11,12 thrombin inhibition, 3 phosphatase inhibition, 9 protease inhibition, 3 or antifungal properties. 5,6,10,13,14 However, there have not been any previous reports of peptides from Theonella that inhibit the human immunodeficiency virus (HIV). The only sponge-* To whom correspondence should be addressed. Michael R. Boyd: phone,