A collection of the marine sponge Dysidea herbacea containing cyanobacterial symbionts from Papua New Guinea yielded a new ketide amino acid, herbamide A (1), along with the previously reported polychlorinated tetrapeptide, dysidenin (2). The structure of 1 was determined by spectroscopic methods. Dysidea herbacea, a common Indo-Pacific sponge, has been extensively studied for its chemistry and biology; its morphology and pigmentation vary considerably, with some specimens harboring cyanobacteria or rod-shaped bacteria and others devoid of prokaryotes. Samples without cyanobacteria are sources of sesquiterpenoids, while "green" forms with Oscillatoria spongiae cyanobacteria are rich in polychlorinated peptides or polybrominated phenols. Recent studies show sesquiterpenoids and chlorinated polypeptides concentrate in sponge or cyanobacteria cells, respectively. We surveyed various D. herbacea forms from Papua New Guinea and report on a sample with new peptide constituents. The D. herbacea (coll. no. 93153), presumed cyanobacteria-rich via TLC-detected chlorophyll, was processed; its dichloromethane fraction (showing polypeptide NMR resonances) was purified by HPLC to yield 1 and 2. Structural elucidation of 1 revealed high chlorination: LRFABMS showed three chlorine isotope distribution, HRFABMS established the molecular formula C₁₆H₂₁Cl₃N₂OS (m/z 394.0442 [MH]⁺). MS and ¹³C NMR APT data indicated NH and an amide moiety (δ 165.6). Seven ¹³C resonances suggested a thiazole ring and two double bonds (accounting for unsaturation). UV (λmax 258) and IR (1672, 1602 cm⁻¹) supported extended conjugation from a dieneamide functionality. ¹H-¹H COSY confirmed the thiazole ring and two acyclic chains (NH-CH(CH₃)₂ and C1-C7). ¹³C NMR chemical shift analysis placed the C13C residue at C2, and HMBC correlations (e.g., C14 to H9, amide NH to C8, vinyl H7 to C8) finalized atom connectivities. The major component 2 was provisionally identified as (-)-dysidenin by comparing NMR/mass spectral data to literature: its [α]D = -68° was close to (-)-dysidenin's -98°, and ¹H NMR shifts matched those of (-)-dysidenin, suggesting identical stereochemistry at C2, C5, and C7. Herbamide A extends the theme of Dysidea chloropeptides (associated with cyanobacterial symbionts), containing a trichlorovaline subunit analogous to those in 2 and (+)-isodysidenin (3). Its nine-carbon side chain resembles the ten-carbon chain of bengarnide metabolites (biogenetically linked to valine + diketide), and its thiazole portion mimics the terminal unit of dolastatin 10. In contrast to the potent cytotoxicity of dolastatins, compound 1 was inactive in the NCI disease-oriented cytotoxicity screen.